ABSTRACTInformation on the hemodynamic effects of vasopressin (AVP) in healthy humans is very limited despite its known importance in body fluid homeostasis and release in pathologic states such as hemorrhage and trauma. Although it is a potent vasoconstrictor in vitro, it does not cause the expected rise in arterial pressure when given systemically to animals with intact baroreflexes. It has been proposed that this is because AVP facilitates baroreflex control of the circulation. In this study, we assessed the effect of infusion of AVP on resting circulatory variables and on the baroreflex control of forearm vascular resistance and heart rate in healthy men. AVP in a dose of 0.4 ng/kg/min, which raised plasma level of AVP to 24 ± 4 pg/ml, a value known to have a significant antidiuretic effect, had little hemodynamic effect, producing only mild bradycardia and a slight increase in central venous pressure. Reflex changes in heart rate during neck suction (- 15 and -30 mm Hg) and neck pressure (+ 15 and + 30 mm Hg) were not altered. Reflex responses to lower body negative pressure and to its release were also unchanged by this dose of AVP. In contrast, a higher dose of AVP (4 ng/kg/min),which raised plasma levels to 290 ± 41 pg/ml, a concentration known to occur as a result of hemorrhagic hypotension and circulatory stresses, did cause hemodynamic changes. There was tachycardia (from 63 ± 2 to 68 ± 2 beats/min), a decrease in pulse pressure (from 62 ± 2 to 53 ± 2 mm Hg), an increase in central venous pressure (from 2.6 ± 0.5 to 4.1 0.4 mm Hg), and surprisingly in view of the known vasoconstrictor effect of AVP, an increase in forearm flow (from 4.4 0.7 to 5.9 ± 1.2 ml/min/100 ml tissue) and a decrease in forearm vascular resistance (from 24 ± 4 to 18 ± 3 U);there was no significant change in mean arterial pressure (from 83 ± 2 to 83 ± 3 mm Hg). Reflex changes in heart rate were unaltered. The maximal vasoconstrictor response in the forearm attained during lower body negative pressure was not influenced by AVP, but the reflex vasodilator response to the sudden release of lower body negative pressure was significantly augmented, vascular resistance falling to 23 ± 4 U before and 13 ± 2 U during AVP. The unanticipated findings in this study include the biphasic changes in heart rate with increasing doses of AVP, the absence of a pressor response, and the vasodilatation in forearm vessels. We speculate that the vasodilatation in the forearm is the result of an increase in central venous pressure and a facilitated baroreflex-mediated withdrawal of sympathetic tone.