The finding is similar to a report in 1991 by Dr. Simon LeVay that a small region of the hypothalamus is twice as large in straight men as in women or gay men. The brain scanning technique used by the Swedish researchers lacks the resolution to see the region studied by Dr. LeVay, which is a mere millimeter or so across. But both findings suggest that the hypothalamus is organized in a way related to sexual orientation.

The new finding, if confirmed, would break ground in two important directions, those of human pheromones and human sexuality.

Mice are known to influence each other's sexual behavior through emission of chemicals that act like hormones on the recipient's brain and so are known as pheromones. Hopes by the fragrance industry, among others, of finding human pheromones were dashed several years ago when it emerged that a tiny structure in the nose through which mice detect many pheromones, the vomeronasal organ, is largely inactive in humans, having lost its nervous connection with the brain.

Researchers interpreted that to mean that humans, as they evolved to rely on sight more than smell, had no need of the primitive cues that pass for sexual attractiveness in mice. But a role for human pheromones could not be ruled out, especially in light of findings that women living or working together tend to synchronize their menstrual cycles.

Some researchers see Dr. Savic's work as strong evidence in favor of human pheromones. "The question of whether human pheromones exist has been answered. They do," wrote the authors of a commentary in Neuron about Dr. Savic's report of 2001.

Dr. Catherine Dulac, a Harvard University biologist who studies pheromones in mice, said that if a chemical modified the function of the hypothalamus, that might be enough to regard it as a pheromone. She said the Swedish study was extremely interesting, even though "humans are a terrible experimental subject." She noted, however, that the researchers used a far higher dose of the armpit chemical than anyone would be exposed to in normal life.

If human pheromones do exist, Dr. Savic's approach may allow insights into how the brain is organized not just for sexual orientation but also for sexuality in general.