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The acute effects of two calcium channel blockers, nifedipine and verapamil, were compared in eight normotensive subjects and eight patients with essential hypertension. Nifedipine 10 mg and verapamil 160 mg orally had no effect on blood pressure of normal subjects, but reduced systolic and diastolic pressures of hypertensive patients to the same extent. The blood pressure reduction caused by nifedipine was more prompt and of lesser duration than that caused by verapamil. In both normal subjects and hypertensive patients nifedipine caused a transient rise in heart rate and plasma renin activity, and plasma catecholamines showed a tendency to increase; verapamil did not affect these variables. Nifedipine induced a marked increase in urine volume and renal sodium excretion in hypertensive patients, with a much smaller change in normotensives. Verapamil did not influence water and sodium excretion in either direction. Thus, this study shows similarities and differences between the effects induced by acute oral administration of the most-used vasodilating calcium antagonists.