Changes in plasma testosterone (T) levels following the placement of a receptive female into a male’s cage and after exposure to the odor of vaginal discharge in the absence of a female were studied in the male hamster. Plasma T was determined by radioimmunoassay. Both treatments produced significant increases in T levels. The degree of interest in the odor was positively related to the males’ baseline T levels. The results demonstrate a direct effect of a sex-related odor on neuroendocrine activity in a male mammal.