The peptide urotensin II (U II) evokes potent vasoconstriction in non-human primates. In human blood vessels studied in vitro variable effects of U II are reported; vasoconstriction, vasodilatation or no response. It is therefore of importance to determine the vascular effect of U II in humans in vivo. U II (0.1 - 300 pmol min(-1)) was infused into the brachial artery of nine healthy volunteers. Changes in forearm blood flow (FBF) were determined by venous occlusion plethysmography. U II induced dose-dependent reduction in FBF. A threshold response was obtained by 1 pmol min(-1), and the highest dose of U II (300 pmol min(-1)) reduced FBF by 31+/-4% (P<0.01). FBF returned to baseline values within 30 min. This study demonstrates that U II produces potent vasoconstriction in humans in vivo.