The objective of the present study was to examine the role of vasopressin in the regulation of LH secretion in the rhesus monkey. The effect of vasopressin administration on basal LH secretion and vasopressin antagonism on stress-induced inhibition of LH secretion were examined. Intracerebroventricular (i.c.v.) infusion of vasopressin (20 µg/h) to chair restrained ovariectomized rhesus monkeys (n = 5) decreased the area under the LH curve by –51.61 ± 13.73 ng/ml/h compared to –8.35 ± 7.11 ng/ml/h following infusion of artificial CSF (aCSF; p = 0.021). This effect was independent of any change in mean arterial pressure. Subsequently, the role of vasopressin in hypoglycemia-induced suppression of LH was examined. Administration of insulin (1 U/kg BW) to chair-restrained ovariectomized rhesus monkeys decreased the area under the LH curve by –60.88 ± 19.77 ng/ml/h. The decrease in LH was significantly different from that observed in aCSF-infused euglycemic controls which exhibited a slight decrease in LH (–8.35 ± 7.11 ng/ml/h; p = 0.036). In contrast, the area under the LH curve was increased slightly (1.42 ± 11.93 ng/ml/h) when insulin administration was combined with i.c.v. infusion of the vasopressin antagonist [deaminopenicillamine1, O-methyl-tyrosine2, arginine8]-vasopressin (120 µg/h; p = 0.013 vs. insulin only). The demonstration that vasopressin administration inhibits LH secretion whereas vasopressin antagonism prevents hypoglycemia-induced LH suppression suggests that vasopressin is a physiological inhibitor of LH secretion in the