The effect of bilateral orchidectomy and subsequent replacement treatment with testosterone (T) on the intermittent pattern of LH release were studied in six adult male rhesus monkeys. Frequent blood samples were withdrawn for extended periods via remote sampling devices that permitted continuous access to the venous circulation with minimal restraint of the animals. Plasma concentrations of LH were determined by RIA, and episodes of LH secretion (pulses) were identified using a computer-executed algorithm. In intact animals, the frequency of episodic LH secretion averaged one pulse every 4.5 h, and this increased within 2 days of castration to approximately one pulse every hour. The acceleration in LH pulse frequency occasioned by orchidectomy was associated with increases in mean plasma LH concentration and an enhancement of LH pulse amplitude. Implantation of acutely orchidectomized animals with T-containing Silastic capsules that resulted in sustained increments in plasma T concentrations in the mid to upper physiological ranges generally resulted, within 10 days, in a partial to complete reversal of the high LH pulse frequency characteristic of the castrated animal. The onset of the T-induced reduction in LH pulse frequency was associated with a marked increase in pulse amplitude. Since it is generally held that a LH pulse is occasioned by, and therefore reflects, a discharge of hypothalamic gonadotropin-releasing hormone, it may be proposed that in the rhesus monkey, the testes impose a decisive retardation on the neural mechanism that governs the timing of intermittent gonadotropin-releasing hormone secretion, and that this modulation of the hypophysiotropic oscillator by the gonad is mediated by testicular T secretion.