Systemic and central haemodynamics were evaluated in 10 lean and 10 obese hypertensive patients (World Health Organization stage I-II) after treatment for 8 weeks with a serotoninergic antagonist, such as ketanserin. Blood pressure and heart rate were recorded and first-pass radionuclide angiocardiography was performed to determine cardiac output, cardiac index and ejection fraction of the left ventricle; total peripheral resistance was also calculated. In both obese and lean patients, ketanserin significantly reduced diastolic (P less than 0.05) and mean (P less than 0.005) blood pressure but no significant changes in systolic blood pressure, cardiac output, cardiac index and ejection fraction were observed in lean and obese hypertensive patients. Total peripheral resistance was significantly (P less than 0.05) reduced in lean patients but in obese hypertensives it was only moderately reduced. It is concluded that monotherapy with ketanserin is effective in treating mild to moderate hypertension in both lean and obese hypertensive patients, without interfering with left ventricular performance.