Plasma concentrations of LH and FSH in immature male rhesus monkeys were determined at weekly intervals from birth until 300 days of age, and the effect of bilateral orchidectomy at 1 week of age on the neonatal pattern of gonadotropin secretion was studied. Plasma FSH and LH concentrations in intact animals were elevated during the first few months of neonatal life but then declined to undetectable values characteristic of the prepubertal state in this species. Neonatal castration, but not sham castration, resulted in a progressive and dramatic rise in circulating gonadotropin concentrations, which plateaued approximately 3 weeks later at values an order of magnitude greater than those observed in intact animals. The hypersecretion of LH and FSH after removal of the gonads was not sustained, however, and by 32 weeks of age plasma gonadotropin concentrations in the agonadal animals were indistinguishable from those in age-matched intact controls. These findings suggest that, in the rhesus monkey, the testes do not play a major role in determining the pattern of gonadotropin secretion throughout neonatal and prepubertal development. They further suggest that a profound inhibitory influence of nongonadal origin impinges upon the neuroendocrine control system governing gonadotropin secretion in the neonate, thereby initiating the diminished secretion of LH and FSH which is characteristic of the prepubertal stage of development in primates.