Ketanserin, an antagonist highly selective for 5-hydroxytryptamine (serotonin) type 2 (S2) receptors, was given as monotherapy in a dose of 40 mg b.i.d. to 24 subjects with mild to moderate essential hypertension. Its effects were evaluated in a placebo-controlled double-blind crossover study. The effect on blood pressure in 18 subjects was monitored by 24-hour ambulatory intra-arterial measurements. Systolic and diastolic intra-arterial pressures were significantly lowered by ketanserin both during the day and at night, whereas heart rate was unchanged. Cuff pressure readings (triplicate measurements) with the London School of Hygiene sphygmomanometer and an automatic device (12 measurements In 1 hour) in the outpatient clinic also showed a significant effect on both supine and standing pressures. No postural hypotension was noted. Ketanserin had no effect on endogenous creatinine clearance, serum cholesterol levels, and the plasma levels of norepinephrine, renin, and aldmterone. The only side effect that was significantly more common with ketanserin than with placebo treatment was an increase in body weight. Ketanserin may have a place in the treatment of mild to moderate essential hypertension.