Plasma levels of radioimmunoreactive LH, FSH, and testosterone (T) were assayed before and after administration of synthetic gonadotropin-releasing hormone (GnRH) to five chair-restrained rhesus monkeys with chronic indwelling venous catheters. Intravenous injection of 1, 5, and 25 microgram or infusion of 1 microgram/min for 25 min of GnRH resulted in a significant increase in plasma levels of LH. However, no significant increases in plasma FSH levels were detected. Plasma levels of T were also elevated after administration of 25 microgram GnRH, but peak concentration of T lagged behind peak levels of LH by approximately 30 min. These studies indicate that the male rhesus responds to GnRH administration by increased secretion of LH, followed by an increase in T levels. A concomitant increase in plasma FSH was not observed after treatment with GnRH in the doses used.