The Harderian glands of Syrian hamsters exhibit a marked sexual dimorphism in cell types and porphyrin production. The glands of male hamsters have two secretory cell types (Type I and II) while the glands of females consist of a single secretory cell type (female Type I) and large intraluminal deposits of porphyrins. Besides androgens, there is evidence that the pineal gland, through the secretion of melatonin, contributes to the maintenance of the "male" and "female" phenotypes. In this study, we investigated the effects of castration, short photoperiods, and pinealectomy on the distribution of secretory cells and porphyrin deposits in the Harderian glands of male Syrian hamsters. Two groups of animals were maintained in long days (14 hr light/day). Hamsters in one group were left intact and those in the other were castrated. Another three groups were maintained in short days (8 hr light/day); these animals were either left intact, castrated, or both castrated and pinealectomized. The duration of the experiment was 5 weeks. Castration of long photoperiod-exposed animals resulted in a significant drop in the number of Type II cells and a large increase in the porphyrin deposits (P < 0.01). However, castrated animals exposed to short photoperiod showed a significant smaller change in both parameters compared with those exposed to long days (P < 0.05). Pinealectomy prevented the effects of short days in castrated animals. No significant changes were observed in the relative number of mitotic figures or in the number of cell nuclei, indicating that the changes observed were due in part to a transformation of Type II into Type I cells. In a second experiment, male hamsters were injected daily either with 25 micrograms of melatonin late in the afternoon or with the saline for 8 weeks. The administration of melatonin resulted in a significant (P < 0.05) increase in the percentage of Type II cells. We conclude that when circulating androgens are very low or absent, pineal melatonin maintains the male phenotype in the Syrian hamster Harderian gland.