AbstractThe ultrastructure of the pineal gland of the fox was examined and compared with that of other mammals. The pineal gland of the fox is composed of two different populations of pinealocytes (I and II). The pinealocytes I were distributed homogeneously throughout the parenchyma, while pinealocytes II were located generally near blood vessels. A Golgi apparatus, granular endo‐plasmic reticulum, mitochondria, lysosomes, centrioles, and cilia were present in both cell populations. A characteristic feature of pinealocytes I was the presence of dense‐core vesicles, presumably of Golgi origin; whereas glycogen deposits and pigment granules were common features of pinealocytes II. In addition to the pinealocytes, the parenchyma contained fibrous astrocytes. The capillaries of the pineal gland of the fox consisted of a nonfenestrated endothelium. Numerous nerve fibers, presumably adrenergic, were observed throughout the parenchyma.