Abstract Intravenous infusion of 150 μg of clonidine produced a prompt reduction of arterial pressure in a group of 8 patients with arterial hypertension. Systolic pressure fell 19 per cent, diastolic 16 per cent, and mean pressure 17 per cent. A second dose of 150 μg of clonidine given 30 min. later produced further significant change of only systolic pressure which was reduced 28 per cent below control. Changes in the pulmonary circulation were similar in the systemic circulation. Clonidine appears to lower blood pressure through reduction in cardiac output and perhaps by some decrease in tone of resistance vessels. The fall in cardiac output is probably caused chiefly by reduction of left ventricular filling pressure. Central inhibition of sympathetic transmission to the venous capacitance vessels may result in pooling of blood peripherally with lowering of the venous return and cardiac output.