As the big survivors among Internet companies mature, they are learning a painful -- and unexpected -- lesson: Staying in the online game requires heavy, constant spending.

That reality is a far cry from the view a decade ago, when the Internet was seen as a relatively cheap place to do business. Since Web start-ups required little investment in physical facilities or labor, many investors argued that the high spending of those early days was the temporary price of pioneering new markets. Companies needing heavy investment -- like retailer Amazon.com Inc., with its big warehouses -- were expected to profit handsomely once their markets were sewn up.

Instead, it is becoming increasingly clear -- on Wall Street and beyond -- that just as established brick-and-mortar companies must maintain capital spending to remain competitive, Web companies, too, must keep spending for workers and customers, as well as for the technology that keeps their businesses humming: computers, servers, networking equipment, data-storage equipment and the like.

That's especially true as Web companies challenge one another's turf in areas like retail and search engines. Meanwhile, telecommunications companies, consolidating and rethinking their business models, are considering ways to charge online businesses for the traffic they send over phone networks, which could add to Web companies' costs.

Last week, for instance, Google Inc. Chief Financial Officer George Reyes warned analysts that the Web-search giant will continue to invest heavily in broadening its business, including a "substantial increase" in capital expenditures, driven by spending on computers, data centers and networking equipment. Mr. Reyes said he expects operating expenses to climb, too, as Google hires more engineers and sales and marketing employees.

Already, Google's research-and-development spending more than doubled in 2005, as did its capital expenditures on property and equipment. Concerns about spending are one reason the company's highflying stock price has retreated some in recent weeks.

Even with that decline, Google shares are trading at a price/earnings ratio of 73. Indeed, investors are showing a strong commitment to Web stocks despite evidence that high spending is becoming a constant. The technology sector boasts a P/E ratio of 22, compared with an average P/E of 18 for stocks in the Standard & Poor's 500 Index. Internet commerce stocks are particularly pricey: Amazon trades at a multiple of 44, for instance, while eBay Inc. trades at a P/E of 51.

Investors say it can be hard to justify numbers like that, but many also argue that the companies hold out the bright prospects of any promising growth stock. "The Internet, we think, is far from mature," says Ken Smith, a portfolio manager at Munder Capital, which is invested in Amazon, Google, Yahoo Inc. and eBay.

Kevin Landis, chief executive of Firsthand Funds, based in San Jose, Calif., is invested in the same stocks, though to minimize risk, none of them except Yahoo ranks in his top 10 holdings in any fund. "The question isn't, Are they spending? The question is, What are they getting for what they're spending?" Mr. Landis says. "Like most growth stocks, a lot of that spending is not just to support the existing business. You're spending for acceleration."

Investment "is simply the nature of the ecosystem that we inhabit," said Expedia Inc. Chairman Barry Diller in his travel company's Feb. 15 conference call. Competition, both from traditional rivals and encroaching adversaries, has raised the stakes for every Internet company, he said. He went on to predict substantially higher technology spending this year for Expedia, which was spun out of IAC/InterActiveCorp last year.

Amazon, one of the oldest Web survivors, spent $451 million on technology and content last year. Some analysts predict that spending will grow about 30 percent this year, to about $587 million. Capital spending is expected to grow to about $225 million from $204 million in 2005.

At online auctioneer eBay, product-development costs rose 36 percent last year to $328.2 million, though they remained flat as a percentage of revenue. In 2004, the jump was 51 percent, to $240.6 million. Yahoo has seen capital expenditures rise as a percentage of revenue in two of the past three years.

Google's sales and marketing expenses jumped 47 percent in the fourth quarter from the third quarter, prompting investor concern that such spending increases could crimp profits. "The spending question was the question of the quarter across all the Internet names," says Robert Peck, a Bear Stearns analyst.

Google shows how capital-intensive a virtual business can be. The Web-search giant's business doesn't require big warehouses. But it still laid out $838 million last year, primarily on computer servers, data centers and networking equipment required to operate its consumer services.

Aiming to bolster usage of its services and compete with Microsoft Corp., Google also is in talks with Dell Inc. to pay fees that could approach $1 billion to install Google software on as many as 100,000 new Dell personal computers over three years, according to people familiar with the matter.

Some investors believe that Google's influence in turn fuels spending at Yahoo and others that want to compete with its services. Yahoo, in a statement, says it bases its spending on what's best for its future. The company also says its rapid growth -- its user base has doubled in the past two years -- has prompted further product-development and capital-spending investments.

The risky nature of the Web itself drives costs higher. In an arena where markets change so rapidly and customers can be fickle, Web companies feel obliged to place many bets on where growth is going to come from -- some of which will fail to pay off.

Last year, eBay made its biggest-ever acquisition by buying Skype Technologies SA, an Internet-phone company, for $2.5 billion. Some analysts questioned the deal as a costly departure from eBay's core business. But spokesman Hani Durzy says eBay paid a fair price for a company that fits in with its core business of helping buyers and sellers communicate.

EBay also has been trying to hold the line on some heavy investments. Its spending in sales and marketing as a percentage of revenue has declined or stayed flat in two of the past three years.

Also getting into the act has been Microsoft. Research-and-development spending at the Redmond, Wash., company has remained fairly constant since 1996, fluctuating between 15 percent and 21 percent of total revenue. But in a recent quarterly filing, Microsoft warned that its MSN Internet unit's "profitability may decline in fiscal year 2006 as investments are made in the development of new applications and services, the search and search monetization platform, and expanding the field sales force."

At a recent investment conference, Microsoft Chief Financial Officer Christopher Liddell said the company is spending more to close the gap with rivals. "Clearly we have a lot of work to do and we are investing aggressively," he said.

One of the Web's biggest spenders is Amazon, which must pay for online costs and maintain inventories of books, CDs and other products. It is "inherently a retailer, but they have to react and behave like an Internet company," says Safa Rashtchy, a Piper Jaffray analyst, who rates the stock "market perform."

Amazon is juggling a variety of investments, from its own Web searcher to digital music and video initiatives, product expansion and services for merchants selling on its site. Recently, Amazon has been talking with major music companies about offering a digital-music-subscription service on an Amazon-branded music player, which could challenge Apple Computer Inc.'s iTunes service and popular iPod device, according to people familiar with the matter. It also has opened two development centers for media initiatives in the San Francisco area, said another person familiar with the company.

At the same time, Amazon has been bulking up. Last year, it hired 3,000 people, more than the 2,659 recruited to Google and the 2,185 who joined Yahoo. "If you look at 2005, it was definitely a year where we increased our cost structure significantly," Amazon Chief Executive Jeff Bezos said on a recent conference call. "We did that very deliberately, and we did it because we see a lot of opportunity going forward. We will start to see the utilization of that cost structure over the next couple of years."

Last month, the company disclosed disappointing fourth-quarter financial results. Rising operating costs -- including a 57 percent increase in technology and content expenses -- hurt quarterly profit, which fell 43 percent from a year earlier to $199 million.

Of late, operating expenses have been increasing faster than sales or gross profits. "Amazon is a company which will perpetually be in investment mode," Mr. Rashtchy says.

But Amazon spokeswoman Patty Smith says the spending is needed to deliver technical innovation to customers. "If you're going to be a leader," she says, "you're not going to get that sitting on your wallet."