Localization of mast cells in the testis and ability of histamine (HA) to stimulate testicular testosterone (T) production in vitro were examined in gonadally active adult golden hamsters, kept under a long daily photoperiod (LD) and in gonadally regressed adult golden hamsters exposed to an inhibitory short photoperiod (SD). In both groups mast cells were present in the capsule of the testes and occasionally in vicinity of intratesticular blood vessels. Histamine stimulated T production in a dose dependent manner in incubations of regressed testes of SD animals, but had no stimulatory effect in the active testes of LD animals. In the incubations of SD testes, the effects of HA were abolished by the H-1 antagonist pyrilamine, but unaffected by the H-2 antagonist cimetidine. In both groups, HA enhanced the stimulatory effects of hCG. These results indicate that HA has steroidogenic activity in the testis and may be a factor involved in the regulation of Leydig cell function.