The pineal gland is a small diverticulum of ependymal cells dorsal to the third ventricle and adjacent to the epithalamus. The pineal gland of birds appears, at least in part, to be directly sensitive to ambient light passing through the cranium. After light information arrives in the brain stem, whether via the inferior accessory optic tract or the medial forebrain bundle, the subsequent pathways to the pineal gland are shared and involve the sympathetic nervous system. To supplement information derived from measurement of enzymes and indoles in the pineal gland, circulating concentrations of blood-borne melatonin can also be quantified, and this approach is especially valuable in larger species. Changes in the pineal gland during evolution leading up to mammals reflect a progressive loss of photoreception by the pineal structure. Hypophysial secretion of prolactin and luteinizing hormone varies among seasons, and in most mammals the seasonal patterns of hormone secretion are regulated by the environmental photoperiod.