A minute cylindrical structure with a dense core, designated as "microcylinder," has been observed within enlarged spaces of mitochondrial cristae in pinealocytes of some adult rats (osmium tetroxide fixed, methacrylate embedded). The microcylinders are 270 to 330 A in diameter and of indeterminate length. Their wall is found to be made up of slender filamentous subunits, probably 6 in number, and surrounds a central filament. The microcylinders are arranged parallel to one another, forming monolayered or, more frequently, multilayered aggregates. Their number within a crista varies considerably. Packets of microcylinders may be seen located in the outer mitochondrial chamber, but are never found in the mitochondrial matrix. They have been observed neither in other cell types of the pineal gland nor in neurons, ependymal, and glial cells of the nearby epithalamic tissue. The origin and nature of the microcylinders are unknown. Glycogen-like particles have been encountered, though very infrequently, accompanying bundles of microcylinders within mitochondrial cristae.