The actions of endothelin-1 (ET-1) on vascular resistances were investigated and characterized in anesthetized pigs in vivo. Intravenously administered ET-1 (2–400 pmol/kg) caused dose-dependent increases in mean arterial blood pressure accompanied by a reduction in heart rate. ET-1 also increased renal, coronary, femoral, and bronchial vascular resistances. The kidney was about 10 times more sensitive to ET-1 than the other regions. The ET-1-induced vasoconstriction in the kidney was attenuated by nifedipine (100$ mUg/kg). The responses to ET-1 in the femoral and bronchial vascular beds were biphasic with an initial decrease followed by an increase in vascular resistance at high doses. It is concluded that ET-1 causes dose-dependent vasoconstriction in several vascular beds in vivo with the most pronounced effects in the renal circulation.