The hypotensive effect of 4-(2'-nitrophenyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxydimethylester (nifedipine, Adalat) was studied in 15 hypertensive patients with chronic renal failure. The decrease of blood pressure lasted for approximately 3 h and maximum decrement was achieved in 1 h after oral administration of nifedipine (10 mg). Hypotensive action of the drug was associated with a constant decrease in total peripheral resistance and an increase in cardiac output, plasma renin activity (PRA), and plasma norepinephrine (noradrenaline). Initial PRA was apparently the major determinant of the rise in PRA since a close correlation was present between the initial value and the increase induced by nifedipine (r = 0.93, p less than 0.05). Marked reduction of blood pressure without noticeable side effects was observed in two cases who had not responded to previous antihypertensive treatment. These data suggest that nifedipine exerts its hypotensive action through peripheral vasodilation and may be an effective drug for treatment of hypertension during chronic renal failure.