The new selective 5-HT2 receptor blocking agent ketanserin was given in a dose of 10 mg intravenously to 12 patients with essential hypertension. It caused a distinct fall in supine systemic arterial, right atrial, pulmonary artery, and pulmonary capillary "wedge" pressures. Cardiac output, renal blood flow, and glomerular filtration rate showed no persistent changes. Thus 5-HT2 receptor blockade caused dilatation of both resistance and capacitance vessels and of the renal vascular bed. Heart rate and plasma concentrations of renin and noradrenaline rose after ketanserin. These data suggest that 5-HT may have a role in maintaining high blood pressure.