The primary aim of this paper is to present a series of experiments designed to study the mechanisms which control the onset of puberty in the rat. A second major goal is to discuss these results in relation to observations in other mammalian and certain non-mammalian species in an attempt to develop a generalized concept for the mechanisms controlling the onset of puberty or reproductive capability. Throughout the animal kingdom regulatory mechanisms for precise control of gonadal differentiation, maturation and function have evolved to insure that reproduction is possible. The degree of complexity of these mechanisms increases as one proceeds from phylogenetically lower animals to mammals (Hagadorn, 1967; Rothballer, 1954). Neurosecretory cells have been identified in all invertebrate phyla that possess a well defined nervous system. Even though such cells do not exist in coelenterates, specialized hypostomal nerve cells do play a role in the initiation of growth, sexuality and regeneration. In echinoderms, such as the star fish, the circumoral ring and radial nerves contain neurosecretory cells which are thought to release substances which stimulate and inhibit the release of ripe gametes. In polychaete and oligochaete worms surgical removal of cerebral ganglia produces premature gamete formation. These regulatory mechanisms relate to what Hagadorn calls a “First order neuroendocrine reflex” that involves a direct action of central nervous system products on the target or gonad. Inhibitory nerves from the subpedunculate lobe of the brain of the octopus regulate the release of gonadotropic hormones from a specialized optic gland. Elaborate regulatory mechanisms