Pinealocytes constitute 90% of the cell population in the pineal organ (Quay, 1965) and are thought to have an endocrine function (Kelly, 1962; Thieblot & Blaise, 1963; Zweens, 1965). Removal of the pineal in prepubertal rats induces gonadal hypertrophy (Kitay, 1954) and Gittes & Chu (1965) reported that the effects of pinealectomy on the oestrous cycle of female rats could be consistently reversed by transplantation of isogeneic pineal tissue. It seems, therefore, that a pineal—gonadal interrelationship exists. This investigation was undertaken to study the cellular changes in the rat pineal gland after gonadectomy. The pineal organ of each member of four groups of rats was studied: (a) A prepubertal group of 24 males and 24 females gonadectomized at 1–2 weeks of age and killed 10–12 weeks later; (b) Twelve males and 12 females were used as controls for group a. They were sham-operated at 10–12 days of age and killed