To evaluate the antihypertensive effects of serotonin antagonist, ketanserin, a daily dosage of 10~40mg (18.9±6.8 mean±SE) was administered to 22 patients with essential hypertension and aged 55??1 years (64±7.7 mean±SE) for 12 weeks. After 12 weeks of ketanserin treatment mean blood pressure decreased from 177.9±11.9 to 15.7±15.3mmHg in systole and from 104.2±7.4 to 88.4±7.9mmHg in diastole (P<0.001). There was no significant change in heart rates with ketanserin treatment. The antihypertensive treatment with ketanserin was effective in 18 patients(81.8%) and ineffective in 4 patients (18.2%). Adverse reactions such as drowsiness (8.7%), edama (4.3%) and weakness (4.3%) were noted, but all were mild and transient. This results suggest that ketanserin is an effective and safe antihypersive agent in the treatment of essential hyperension.