The present study reexamined the role of the ovary in determining the prepubertal hiatus of gonadotropin secretion in the rhesus monkey. Day- and nighttime blood samples were obtained weekly from neonatally (7-10 days of age) ovariectomized and intact monkeys from birth until 3 yr of age. In the intact monkeys, plasma FSH levels increased during the first month of life, remained elevated until approximately 3 months of age, and then decreased to become undetectable by 7 months of age. Thereafter, plasma FSH remained undetectable until approximately 19 months of age, at which time it again increased to detectable concentrations. In animals ovariectomized as neonates, the developmental pattern in FSH secretion was similar to that in intact animals, but, quantitatively, mean plasma FSH concentrations in the agonadal females were greater than those in the intact control group at all times. Circulating daytime LH concentrations in intact animals were generally below the sensitivity of the assay during the neonatal and prepubertal phases of development, but after 27 months of age, this plasma hormone was measurable on occasion. In neonatally ovariectomized monkeys, daytime LH was elevated during the first month of life, undetectable between 2-20 months of age, and then rose into the adult range by the end of the study. Nocturnal plasma FSH and LH concentrations in agonadal monkeys were generally greater than those during the day at all stages of development. Of particular note was the finding that during the prepubertal hiatus in gonadotropin secretion, when daytime LH levels were mostly immeasurable, nighttime levels of this gonadotropin were consistently elevated. The hypersecretion of gonadotropin during prepubertal development in agonadal animals also occurred when ovariectomy was performed at 61-62 weeks of age. These findings demonstrate that in the female monkey, the open loop activity of the GnRH pulse generator during juvenile development is only partially suppressed, and the ovary contributes significantly to the prepubertal restraint on gonadotropin secretion. We also report the serendipitous finding that a precipitous, albeit transient, decline in circulating gonadotropin concentrations occurred in juvenile monkeys after separation from their mothers and relocation to individual cages. This suppression, which was accompanied by elevated plasma cortisol levels, was apparently not related to any impairment in growth.