Castrated adult male golden hamsters maintained on either a stimulatory (LD 14:10) or a nonstimulatory (LD 6:18 ) light cycle for 10 weeks following surgery were implanted sc with empty or testosterone-filled Silastic capsules of various lengths. Capsules which maintained circulating androgen levels between 0.5 and 1.7 ng/ml reduced serum levels of LH and FSH in castrated animals exposed to LD 6:18 but not in animals maintained on LD 14:10. When serum androgen levels were maintained at about 3 ng/ml, serum gonadotropin levels were reduced 30--35-fold in nonphotostimulated hamsters but only 2--3-fold in photostimulated animals. Maintenance of serum androgen levels at 14--17 ng/ml induced a maximum decrease in serum LH and FSH in animals exposed to either photoperiod. These results indicate that exposure to short days for 10 weeks renders the hypothalamic-hypophyseal axis of the castrate hamster extremely sensitive to the negative feedback effect of testosterone. This alteration in steroid feedback sensitivity may be one way in which the photoperiod acts to alter neuroendocrine-gonadal activity in seasonally breeding animals.