The intracranial epiphysis of the adult frog, Rana pipiens, has been examined by electron microscopic cytochemistry and radioautography. Acid phosphatase is localized within vacuoles of macrophages free in the lumen, in subspherical vesicles of the ellipsoid portions of photoreceptive cell inner segments, and in occasional heterogeneous cytoplasmic inclusions of the supportive cells.

The distribution of radioactivity in the epiphysis at intervals of 1, 3, 5, 9, 14, 25, and 60 days following injection of tritiated leucine, as determined by quantitative radioautography, is consistent with an hypothesized process of continual renewal of photoreceptive cell outer segments. The pattern of radioautographic labeling of the pineal photoreceptors, which resembles more closely that of retinal cones than rods, is correlated with previous morphologic and electrophysiological studies of these cells.

The radioautographic and cytochemical data suggest that macrophages within the epiphyseal lumen are involved in phagocytosis and, ultimately, in digestion of degenerate outer segments. They may perform a function similar to that of pigment epithelial cells of the lateral eye retina.