The Harderian glands of the golden hamster exhibit a sexual dimorphism manifested by acinar cells of one type and the presence of porphyrin granules in the female, and by acinar cells of two types and the absence of porphyrin in the male. Castration of male hamsters produces Harderian glands of the female type. In the present study, it was found that a similar effect is not produced by pineal-induced testicular regression and that blinding inhibits the usual castration-induced change in the Harderian glands. Combinations of procedures involving castration, blinding, and pinealectomy of adult male hamsters indicate that the pineal gland is responsible for this inhibition. The findings support the hypothesis that elevated gonadotropin levels are responsible for producing the female type of Harderian glands in castrated males.