Exposure to short day lengths for 9 weeks renders the hypothalamic-pituitary axis of the castrate hamster extremely responsive to the negative feedback effect of exogenous testosterone. The systemic and/or local conversion of testosterone to 5α-dihydrotestosterone (5α-DHT) or 17β-estradiol (E2) is considered to be an important step in its action on target tissues. The present study was designed to determine if the photoperiod can induce changes in the sensitivity of the hypothalamic-pituitary axis to the inhibitory effects of 5α-DHT or E2. 5α-DHT-filled Silastic capsules that were 2, 4, or 8 mm long greatly reduced serum LH and FSH levels in castrates that were exposed to a nonstimulatory LD 8:16 light cycle, but not in animals exposed to a stimulatory LD 14:10 light cycle. E2 capsules that were 1, 2, 4, or 8 mm long greatly reduced serum LH and FSH levels in castrates exposed to LD 8:16, but not in animals exposed to LD 14:10. Serum gonadotropin levels were reduced in all of the animals receiving the larger 5α-DHT (20 or 50 mm long) or E2 (20 mm long) capsules, irrespective of photoperiod. Thus, the photoperiod can alter the sensitivity of the gonadotropin control center to the negative feedback effect of both the 5α-reduced derivative of testosterone, 5α-DHT, and the aromatized metabolite of testosterone, E2. These results suggest that testicular secretions besides testosterone and/or extratesticular conversion of testosterone to its metabolites may be involved in the photoperiodic inhibition of the hamster reproductive