The Harderian gland of the female golden hamster contains an average of forty times more mast cells in the interstitial tissue than the male gland. A similar sex difference occurs in the connective tissue capsule of the gland. Mast cell numbers in the male Harderian gland increase greatly between two and five months after castration, reaching levels comparable to those found in the female. This postcastrational rise in mast cell numbers is prevented by androgen administration; furthermore, if mast cell numbers are allowed to increase in castrates, subsequent androgen administration will reverse the rise. There is no obvious relationship between mast cell numbers in the Harderian gland of the female hamster and the porphyrin content of the gland.