Calcium antagonists have been used for the acute treatment of hypertension. Among these compounds, diltiazem induces the least tachycardia in response to acute administration. The effects of this agent, in a dose of 0.3 mg/kg given intravenously, on arterial pressure, heart rate and plasma renin activity were assessed in 10 patients with benign moderate to severe essential hypertension; nifedipine (10 mg sublingually) in 10 patients or 5% glucose (placebo) in 4 patients was used as a control. As compared with the 5% glucose group, diltiazem caused a persistent fall in diastolic pressure during 60 minutes, but only a transient decrease in systolic arterial pressure and heart rate; plasma renin activity was unchanged. Nifedipine caused a persistent fall in both systolic and diastolic pressure and a slight but significant increase in plasma renin activity, but did not modify heart rate. The diltiazem-induced decrease in systolic pressure was significantly smaller than the pressure changes caused by nifedipine. Variations in plasma renin activity between the two calcium antagonists were not significant. These findings suggest that in patients with hypertension diltiazem exerts a less marked acute antihypertensive effect than nifedipine.