WASHINGTON -- Lesbians' brains react differently to sex hormones than those of heterosexual women, new research indicates.

That is in line with an earlier study which had indicated that gay men's brain responses were different from straight men -- though the difference for men was more pronounced than has now been found in women.

Lesbians' brains reacted somewhat, though not completely, like those of heterosexual men, a team of Swedish researchers said in today's edition of Proceedings of the National Academy of Sciences.

A year ago, the same group reported findings for gay men that showed their brain response to hormones was similar to that of heterosexual women.

In both cases, the findings add weight to the idea that homosexuality has a physical basis and is not learned behavior.

"The important thing is to be open to the likely situation that there are biological factors that contribute to sexual orientation," said Dr. Sandra Witelson, an expert on brain anatomy and sexual orientation at McMaster University in Hamilton, Ontario, not part of the research team.

The team led by Dr. Ivanka Savic at the Stockholm Brain Institute had volunteers sniff chemicals derived from male and female sex hormones.

These chemicals are thought to be pheromones -- molecules that trigger defense and sex in animals.

Heterosexual women found the male and female pheromones equally pleasant, while straight men and lesbians liked the female pheromone more than the male one.

Men and lesbians also found the male hormone more irritating than the female one, while straight women were more likely to be irritated by the female hormone than the male one.

The brains of all three groups were scanned when sniffing male and female hormones and a set of four ordinary odors.