BACKGROUND: Arginine vasopressin (AVP) administered intra-arterially to normal volunteers exerts a biphasic effect on forearm blood flow when the effect is extrapolated from plethysmographic measurements. OBJECTIVE: To assess whether the high-dose AVP-induced increase in forearm blood flow could be confirmed when calculating blood flow from continuous radial artery diameter and flow velocity recordings obtained by using a high-resolution echotracking device combined with a Doppler system. METHODS: Increasing doses (0.04-0.8 ng/kg per min) of AVP were infused into a brachial artery of seven normal male volunteers (aged 21-33 years). Forearm blood flow derived from venous occlusion plethysmography was assessed simultaneously with proximal radial artery blood flow calculated from luminal area and flow velocity measurements. RESULTS: Confirming previous reports, plethysmography showed an increase in global forearm blood flow by > 100% with AVP concentrations > or = 0.2 ng/kg per min. In contrast, direct measurements of lumen diameter and blood flow velocity in the radial artery revealed a marked dose-dependent vasoconstriction with a > 30% decline in blood flow at the highest AVP concentration. CONCLUSIONS: The discrepancy between the two measurements suggests that AVP has a dual effect on forearm haemodynamics. At high AVP concentration, the muscle blood flow increase predominates over the vasoconstriction in the skin circulation. Furthermore, this study strongly suggests a heterogeneity of the vascular response to vasomediators by showing that opposing responses exist not only between resistive and conduit vessels but also between conduit arteries of a common vascular bed.