The pineal gland of the hamster (Mesocricetus auratus) elaborates a strong antigonadotrophic substance; the gonads of hamsters blinded or exposed to darkness undergo total involution in about 6 weeks, an effect abolished by pinealectomy (Reiter, 1968). Specifically, how does the pineal hormone reach the neural site at which it acts: by secretion into the circulatory system or into the cerebrospinal fluid? The latter seemed unrealistic because the rodent pineal, attached to the epithalamus by a long stalk (Kappers, 1965), is far from the third ventricle and could ostensibly only release material into the subarachnoid space or the vascular system. While studying the fine structure of hamster pineals (Sheridan & Reiter, 1968) obtained by established techniques of pinealectomy or by removal of the gland without preserving stalk or meningeal relationships, we have seen characteristic ependymal cells, specifically cells similar to those associated with the choroid plexus, adhering to the surface of