The reproductive systems of mammals are anatomically and physiologically altered by the pineal gland. Characteristically, pineal removal accelerates gonadal growth in immature animals and causes a transient modest enlargement of the reproductive organs if the operations are done after adulthood is attained. Light deprivation renders the pineal strongly antigonadotropic. In addition, there are several conditions, e.g., androgen-sterilization and anosmia, which potentiate, probably indirectly, the gonad-inhibiting capability of the rat's pineal. The chronic administration of pineal extracts or pineal substances has effects opposite to pinealectomy, i.e., they restrict gonadal development or growth. The pineal is possibly important in adjusting the level of reproductive activity to seasonal changes in environmental conditions.