Essential hypertension is a multifactorial disorder mediated by multiple mechanisms. Serotonin may cause vasoconstriction either directly or indirectly by amplifying the actions of vasoconstrictor substances. Clinical trials indicate that ketanserin, a serotonin antagonist, is effective in treating hypertension. It is desirable for an antihypertensive agent to enhance or preserve renal hemodynamic function. In this study, we evaluated the acute (1 week) and chronic (8 weeks) effects of ketanserin (20-40 mg twice a day) on glomerular filtration rate, renal plasma flow, and sodium excretion in patients with uncomplicated hypertension. Patients with untreated diastolic blood pressure (greater than 90 mm Hg) received either ketanserin or a placebo during an 8-week double-blind trial. Findings demonstrated that patients treated with ketanserin showed an increase in renal plasma flow compared with those who received placebo. Sodium excretion remained unchanged, indicating the absence of sodium retention during therapy. These results show that ketanserin lowers blood pressure in essential hypertension while preserving renal hemodynamics and function.