In gonadectomized animals, pulses of LH are secreted concurrently with pulsatile hypothalamic GnRH and it is hypothesized that pulses of GnRH are either driven or modulated by episodes of catecholamine release. The objective of this study was to determine if the alpha-adrenergic antagonist phentolamine (PHEN) can simultaneously block the release of GnRH and LH in ovariectomized (OVX) rhesus macaques. In Exp 1, simultaneous peripheral blood and mediobasal hypothalamic push-pull perfusion (PPP) samples were collected remotely at 10-min intervals for 24 h via a swivel/tether device in eight conscious, freely moving OVX rhesus monkeys. Phentolamine was continuously infused iv for 6 h at the rate of 4 mg/kg BW.h in five animals and 20 mg/kg BW.h in three animals. Infusion started at 6 h after the commencement of PPP. Sampling of PPP and blood continued for 12 h after the cessation of PHEN infusion. Exp 2 was carried out to determine if PHEN affects pituitary responsiveness to exogenous GnRH under conditions similar to those in Exp 1. Exogenous GnRH (5 micrograms, iv) was injected as a single bolus at 10-h intervals before, during, and after either a saline (4 ml/h for 6 h) infusion or, 3 weeks later, a PHEN infusion (4 mg/kgBW.h for 6 h) in three OVX females. The results of Exp 1 show that pulsatile patterns of hypothalamic GnRH and LH were either dampened or abolished by PHEN infusion. During the recovery period after PHEN infusion, pulse amplitudes of LH were enhanced, but pulse amplitudes of endogenous GnRH did not differ, as compared to those of corresponding LH and GnRH before infusion of PHEN. Data from Exp 2 suggested that the alpha-adrenergic blocking agent had no effect on the pituitary LH response to exogenous GnRH administration. These results directly support the hypothesis that adrenergic neuronal activities are critical for the pulsatile release of hypothalamic GnRH which governs the pulsatile release of LH in gonadectomized animals.