Early signs that those economic stimulus checks are putting money into the nation's cash registers surfaced yesterday as discounters such as Wal-Mart and Costco posted sales gains for the month of May.

But the rising price of groceries and gas may be keeping consumers from having much fun with the extra cash. More than one chain reported slow sales in jewelry and various clothing categories while a survey found fewer people than expected putting the money toward savings or big-ticket items such as TVs or vacations.

"May came in better than expected," said Michael P. Niemira, chief economist for the New York trade group International Council of Shopping Centers. "But it is very clear that consumers are spending in a conservative manner as the lift largely came from an increase in sales in the wholesale, drugstore and discount sectors."

The trade group calculated chain store sales rose 3 percent at locations open at least a year, based on results from almost 40 retailers.

Wal-Mart management cited the tax rebate checks as a benefit to its 4 percent rise in sales at established stores, while Costco Wholesale Corp. had a 9 percent bump. Officials there noted that without the rise in the price of gas sold through its pumps that number would have been around 5 percent.

Shoppers trying to save money are shifting where they shop, according to TNS Retail Forward, a consulting group in Columbus, Ohio. One-fifth of primary household shoppers have changed where they go for groceries and other essentials, according to a survey of 4,000 households done the last week of May.

That may help explain why Target reported a 0.7 dip in same-store sales, according to Retail Forward's analysis. Convenience stores and discounter Kmart also are getting hurt by such shifts.

Other merchants being bypassed may be department store chains such as J.C. Penney, Kohl's, Saks and Bon-Ton, all of which reported a drop in sales at established stores. Specialty retailer American Eagle Outfitter reported a 9 percent dip in same-store sales, which met the company's dampened expectations.

Just as retailers have had to move more cautiously than they originally planned, more than a few consumers have reconsidered their spending plans since Congress approved the tax rebates earlier this year.

When Retail Forward surveyed consumers in February, 34 percent planned to save their windfalls. By May, only 29 percent were planning to save.

Back in the winter, 20 percent dreamed of using their tax rebate toward a special purchase, such as a vacation, jewelry or consumer electronics items. By spring, that number had slid to 14 percent.

Just how long any lift from the stimulus checks will last is a matter of debate. "The retail numbers suggest those dollars are starting to have an impact that should last for several months as more shoppers get their rebates," said Frank Badillo, senior economist at Retail Forward.

Management at J.C. Penney was less upbeat on the issue, as a spokeswoman made clear on the company's recorded monthly sales commentary: "While the stimulus checks may provide some boost to consumers' discretionary spending over the next few months, we expect any such benefits will be modest and temporary."