The antihypertensive effect of the combination of ketanserin, a new antiserotonergic agent, and thiazide has been evaluated in 35 patients with arterial hypertension of mild to moderate degree in the greater than 50-year-old age group. Twenty patients were given ketanserin (20 mg) + hydrochlorothiazide (25 mg) (treatment A) while the others were given ketanserin (40 mg) + hydrochlorothiazide (12.5 mg) (treatment B) once daily, for a period of 6 weeks. Twenty-four-hour blood pressure, measured by an automatic recorder, was significantly reduced by both combinations. In particular, treatment A reduced blood pressure from 169 +/- 15/95 +/- 6 mm Hg before treatment to 146 +/- 11/83 +/- 8, 149 +/- 13/82 +/- 10, 143 +/- 12/81 +/- 9, and 151 +/- 14/84 +/- 7 mm Hg at 2, 6, 8, and 24 h, respectively, after the last dose of drug. With treatment B, blood pressure was reduced from 167 +/- 11/97 +/- 7 mm Hg before treatment to 152 +/- 12/89 +/- 8, 151 +/- 15/85 +/- 8, 150 +/- 16/86 +/- 8, and 158 +/- 13/91 +/- 7 mm Hg at 2, 6, 8, and 24 h, respectively. Heart rate was not affected by both treatments despite the fact that ketanserin has been proved to induce a marked vasodilation. Cardiac workload (systolic blood pressure X heart rate) was slightly reduced by the treatments. Treatment A only induced transient dizziness after the first dose of drug; treatment B, on the other hand, induced drowsiness and more marked dizziness, which in one case was also observed after repeated doses of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)