The pineal complex of rodents is made up of a pineal gland which developmentally always originates from the area between the habenular and posterior commissure and a pineal sac which is continuous with the choroid plexus of the third ventricle. At the light-microscopic level, this sac appears to be identical to the choroid plexus.The pineal sac abuts the deep and superficial pineal glands of the golden hamster. In the PET mouse, gerbil, kangaroo rat, and Chinese hamster, the sac is contiguous with only small areas of the pineal gland. This sac never abuts true pineal parenchyma in the albino rat. The variability of the relationship between this sac and pineal parenchyma indicates that the sac may not be the main physiological route of pineal secretion. Venous drainage of the pineal gland consistently seemed to be into the superior sagittal sinus by means of the vena cerebri magna.