This chapter discusses pineal gland, where the proximal part of the pineal is closely related to the third ventricle, and hence to the cerebrospinal fluid. Histologically, the endocrine cells of the pineal, called pinealocytes, are arranged into cords or lobules. The pineal has a rich blood supply. Phylogenetically, pinealocytes are photoreceptor cells which have lost their outer segment. The principal secretory product of the pineal gland is melatonin. Melatonin secretion shows circadian rhythmicity synchronous with the light-dark cycle. During the midmenstrual period, when gonadotropin levels are at their highest, the nocturnal rise in melatonin level is less marked. The survival of hibernating animals in the winter months depends on their ability to reduce their metabolic requirements. Light suppresses pineal secretion, whereas the secretion increases in darkness. Discovery of the actions of melatonin has helped in understanding several disorders as well as suggested their treatment. Melatonin is a putative sleep inducing substance.