In 10 patients with primary arterial hypertension of mild or moderate degree, ketanserin, a competitive antagonist of serotonin receptors, was given for a period of 4 weeks, 40 mg twice daily. In a control group, patients were given 100 mg twice daily of metoprolol for 4 weeks for each treatment. A randomized double-blind crossover model was used. Blood pressure and heart rate were measured at rest and during exercise testing on a bicycle; peripheral blood flow was measured by strain-gauge plethysmography. A slight reduction in resting systolic and diastolic blood pressure without change in heart rate was observed during treatment with ketanserin. Cardiac workload during exercise test did not change over the observation period. A slight increase in resting blood flow to the lower limbs, with a decrease in peripheral resistance was demonstrated by strain-gauge plethysmography.