The squirrel monkey (Saimiri boliviensis boliviensis) has a well-defined breeding season during which adult males undergo androgen-dependent morphological changes, with acquisition of active spermatogenesis. To assess the hormonal events of this annual cycle, blood samples were obtained weeklyfromtenadult males,andserumwasassayedfortestosterone(T), androstenedione (AA), and dehydroepiandrosterone (DHEA). A significant seasonal variation was noted in mean serum T (P < 0.02), AA (P < 0.02), and DHEA (P < 0.001) concentrations. Mean AA concentrations increased from a nonbreeding season nadir of 91.4 f 12.9 ng/ml (mean f standard error) to a prebreeding concentration of 139 f 10.5 ng/ml and breeding season peak of 167.5 f 15.4 ng/ml (P < 0.05). Mean DHEA concentrations increased from a nonbreeding season nadir of 8.3 & 0.8 to a breeding season peak of 14.3 k 1.2 (P < 0.001). Mean T levels in the nonbreeding (52.2 & 11.6 ng/ ml) and prebreeding season (48.6 f 7.4) were similar. However, T significantly increased during the breeding season to 103.5 k 12.8 ng/ml 8 < 0.05). Progressive changes in body weight and morphology paralleled the rise in serum AA levels. The pattern of peripheral serum androgen concentrations throughout the year would suggest annual activation of the hypothalamic-pituitary-adrenal and/or hypothalamic-pituitary-gonadal axes.