In the golden hamster, prolonged exposure to short days (i.e. less than 12.5 h of light/day) induces testicular regression that has usually, but not always, been reported to be accompanied by a decrease in mean serum luteinizing hormone (LH) levels. As LH appears to be released in a pulsatile pattern in most mammalian species, we hypothesized that individual variations in serum LH levels over time may obscure significant changes in pituitary LH release in hamsters exposed to short photoperiods. Therefore, we examined the effects of short day exposure on pulsatile LH release in castrated and intact golden hamsters. Intact and castrated male golden hamsters were maintained on long days (light :dark 14:10 h) or transferred to short days (light:dark 6:18 h). Nine to eleven weeks after transfer, animals from all four groups were fitted with intra-atrial cannulas and bled at 10-min intervals for 6 h. Exposure to short days inhibited LH release in the intact male hamsters. All 6 of the males maintained on long days had pulsatile LH release while only 1 of the 4 intact males maintained on short days showed a pulse of LH during the 6-hour bleeding session. All of the castrates on short days had pulsatile patterns of LH release that were similar in amplitude and frequency to those of the castrates on long days. Castrates on long days had significantly greater LH pulse frequency and amplitude than intacts on long days. From these data we conclude that the pulsatile nature of pituitary LH release in the golden hamster mandates frequent sampling over several hours for accurate determination of serum LH levels and for determining the effects of photoperiod on pituitary gonadotropin release.