The acute cardiovascular effects of oral felodipine (0.05 mg/kg and 0.1 mg/kg) were studied using radionuclide methods in 14 hypertensive patients. Eight were previously untreated and 6 had been treated with atenolol 100mg daily for a least 1 month. The maximal effects were observed 60 minutes after the first oral dose and no greater effect was observed with the higher dose. Felodipine caused a reduction in systemic vascular resistance, with a fall in blood pressure and an increase in cardiac output and left ventricular ejection fraction. Those responses were presumably mediated by the reduction in afterload, as they were not modified by pretreatment with atenolol. There were no changes in venous capacitance and the overall pattern of response was similar to that noted with hydralazine. Thus, in hypertensive subjects felodipine acts as a potent arteriolar vasodilator. The results suggest the drug may be an effective means of controlling hypertension, particularly when given in combination with other antihypertensive therapy.