Nifedipine is a calcium-channel antagonist with effective antihypertensive activity and has been suggested for the treatment of high blood pressure as an alternative to vasodilators. The aim of this study was to define the acute effect of nifedipine and in particular the dose-effect relationship, effects on veins, influence on adrenergic reflexes, and effectiveness on hypertension according to severity and etiologic type. The effects of nifedipine on blood pressure, heart rate, forearm blood flow, peripheral vascular resistance, orthostatic and cold reflexes, and venous tone were examined in 45 patients with hypertension of different etiologies (essential, renovascular, and renal parenchymal) and different World Health Organization grades. The antihypertensive effect was dose dependent, but a dose of 20 mg has nearly maximal activity with acceptable side effects. The drug acts by lowering peripheral vascular resistance, and this lowering is directly related to baseline values; therefore, the antihypertensive effect increases with severity of the hypertension. Nifedipine had the same effect in all three etiologic groups of hypertension studied. The drug seems to increase venous tone, since it caused venoconstriction when locally injected in hand veins. Nifedipine did not alter adrenergic reflexes induced by both cold application and standing and was well tolerated. In conclusion, the calcium antagonist nifedipine for its characteristics of action, at least in acute administration, seems to be a useful alternative in the treatment of various forms of hypertension.