Adaptationist models of human mating provide a useful framework for identifying subtle, biologically based mechanisms influencing cross-gender social interaction. In line with this framework, the current studies examined the extent to which olfactory cues to female ovulation--scents of women at the peak of their reproductive fertility--influence endocrinological responses in men. Men in the current studies smelled T-shirts worn by women near ovulation or far from ovulation (Studies 1 and 2) or control T-shirts not worn by anyone (Study 2). Men exposed to the scent of an ovulating woman subsequently displayed higher levels of testosterone than did men exposed to the scent of a nonovulating woman or a control scent. Hence, olfactory cues signaling women's levels of reproductive fertility were associated with specific endocrinological responses in men--responses that have been linked to sexual behavior and the initiation of romantic courtship.