Suppression of testicular activity in hamsters and voles exposed to constant darkness or short photoperiod is associated with reduced ability of the testes to convert C21 steroid precursors to C19 androgens. In the present study, we have examined the time course of changes in testicular secretion of progesterone and testosterone in vitro in adult male golden hamsters exposed to short photoperiod. Gradual decrease in testicular weight after 1, 2, and 3 months of exposure to short photoperiod was accompanied by significant increase in the amount of progesterone released per milligram of testicular tissue in response to gonadotropin stimulation. In contrast, testosterone response to gonadotropic stimulus progressively decreased. These results suggest that photoperiod-related gonadal atrophy may be accompanied by reduced activity of the 17 alpha-hydroxylase: C17,20-lyase enzyme complex in the testes, and that seasonal transitions between the states of reproductive activity and quiescence involve changes in both the amount of steroids produced by the testes and the relative proportions of testosterone and its precursors.