The ultrastructure of the pineal parenchymal cells, pinealocytes and glia-like cells, of the Chinese hamster (Cricetulus griseus) is described. Pinealocyte nuclei contain spindle-shaped inclusions consisting of bundles of closely packed parallel filaments of about 8 nm in thickness. Mitochondria contain lamellar and tubular cristae. Smaller and larger mitochondria are usually localized separately in individual pinealocytes. Flattened mitochondria contain two to four closely arranged, unusual lamellar cristae, which lie parallel to the surface of the mitochondria. These cristae exhibit particular structures. Rough endoplasmic reticulum is frequently found in close association with such flattened mitochondria. The pinealocyte cytoplasm contains stacks of flattened cisternae, which are continuous with both smooth and rough endoplasmic reticulum. Pinealocyte cell bodies and processes contain granulated vesicles. Although larger granulated vesicles measuring up to 300 nm in diameter occasionally occur, smaller granulated vesicles (about 100 nm in diameter) and larger ones are usually localized separately in individual cells. Vacuoles containing flocculent material are found in groups in close association with rough endoplasmic reticulum. Glia-like cells with darker cytoplasm and nuclei lie surrounding pericapillary or intercellular spaces. The glia-like cells are characterized by the presence of abundant lipid droplets and occasional pigment granules. Bundles of filaments run parallel to the long axis of the processes of the glia-like cells.