In a study using a brain imaging technique, Swedish researchers found that gay men and straight women respond the same to odours that may be involved in sexual arousal.

The study found that homosexual men's brains responded more like those of women when the men sniffed a chemical from the male hormone testosterone, a key component of male sweat.

The new research may open the way to studying human pheromones, as well as the biological basis of sexual preference.

Pheromones, chemicals emitted by one individual to evoke some behavior in another of the same species, are known to govern sexual activity in animals. But experts differ as to what role, if any, they play in making humans sexually attractive to one another.

The researchers looked at 36 subjects, an even mix of straight men, gay men and straight women. All the subjects were healthy, not-medicated, right-handed and HIV negative.

By scanning their heads as they were subjected to routine odours such as lavender or cedar, researchers found all of them reacted in the parts of their brains designed to handle smells.

But when the gay men and straight women were exposed to the smell of testosterone, the part of their brains that deals with sexual response was activated.

The brains of straight men responded to testosterone as a smell only but reacted sexually to the female hormone estrogen.

The study is published in today's issue of The Proceedings of the National Academy of Sciences, by Dr. Ivanka Savic and colleagues at the Karolinska Institute in Stockholm.

Sandra Witelson, an expert on brain anatomy and sexual orientation at McMaster University in Hamilton, Ontario, described the research as "one more piece of evidence.... showing that sexual orientation is not all learned."

Witelson, who was not part of the research team, said the findings clearly show a biological link in sexual orientation.

In a separate study looking at response to body odours, researchers in Philadelphia found sharp differences between gay and straight men and women.

"Our findings support the contention that gender preference has a biological component that is reflected in both the production of different body odours and in the perception of and response to body odours," said neuroscientist Charles Wysocki, who led the study.

It's hard to see how a simple choice to be gay or lesbian would influence the production of body odour, he said.

Wysocki's team at the Monell Chemical Senses Center studied the response of 82 heterosexual and homosexual men and heterosexual and homosexual women to the odours of underarm sweat collected from 24 donors of varied gender and sexual orientation.

They found that gay men differed from heterosexual men and women and from lesbian women, both in terms of which body odours gay men preferred and how their own body odours were regarded by the other groups.

Gay men preferred odours from gay men, while odours from gay men were the least preferred by heterosexual men and women and by lesbian women in the study.

Their findings will be published in the journal Psychological Science in September.

With files from The Associated Press