1. The aim of this study was to examine whether arginine vasopressin modulates the vasoconstricting action of phenylephrine in human forearms. 2. In seven healthy subjects, we determined the percentage increases in forearm vascular resistance evoked by intra-arterial infusion of phenylephrine at graded doses during simultaneous intra-arterial infusion of saline or arginine vasopressin at two doses. Similarly, in another seven subjects, we examined the effects of intra-arterial infusions of saline or angiotensin II on the vasoconstricting action of intra-arterial phenylephrine. 3. Arginine vasopressin and angiotensin II caused small, but insignificant, increases in baseline forearm vascular resistance. Arginine vasopressin at the two doses significantly attenuated the percentage increases in forearm vascular resistance evoked with phenylephrine at graded doses. Angiotensin II did not alter the forearm vascular responses to phenylephrine. 4. Intra-arterial infusion of arginine vasopressin at doses of 0.6 and 1.8 ng/min raised the plasma arginine vasopressin concentration in the venous effluents from 1.6 +/- 0.3 (control) to 4.0 +/- 0.9 and to 16.4 +/- 6.9 pg/ml, respectively. 5. These results suggest that arginine vasopressin at physiological concentrations with a minimal direct effect on resistance arteries attenuates the vasoconstricting action of phenylephrine in human forearms.