Berkeley - A chemical in male sweat can boost mood, brain activity and sexual arousal in heterosexual women, according to a new study released just in time for Valentine's Day.

The study offers the first direct evidence that humans secrete a scent that can affect the physiology of the opposite sex, said researchers at the University of California, Berkeley. Their findings were published this week in The Journal of Neuroscience.

"This is the first time anyone has demonstrated that a change in women's hormonal levels is induced by sniffing an identified compound of male sweat," said study leader Claire Wyart, a postdoctoral fellow at UC Berkeley. "There is much more going on than we think when we are smelling body odor."

The study conducted last year involved 48 undergraduate women who took 20 sniffs from a bottle containing androstadienone, a compound found in male perspiration and other bodily secretions.

The researchers measured the women's levels of the stress hormone cortisol and compared them to the women's responses to a control odor. Cortisol levels in the women rose within about 15 minutes of inhaling the androstadienone scent and remained elevated for more than an hour, UC Berkeley researchers found.

They also discovered that blood pressure, heart rate and breathing increased, mood improved and sexual arousal was boosted.

While the compound can make women feel more positive and sexually aroused, it's still unclear how it affects their behavior, Wyart said.

"Humans are more complex," she said. "You cannot expect them to have stereotypical responses like rodents."