The acute effect of a single oral dose of isradipine 5 mg on blood pressure, renal haemodynamics, electrolyte excretion and plasma renin activity was studied in 10 healthy males. Isradipine did not produce a significant change in systolic or diastolic blood pressure, and glomerular filtration rate, renal plasma flow, renal vascular resistance, and urinary albumin excretion remained constant. There was a marked natriuretic and diuretic effect about 1-3 h after isradipine. Plasma renin activity showed a slight, insignificant increase 1 h after dosing. Uric acid clearance and beta 2-microglobulin excretion showed no significant changes, despite an increase in sodium clearance, suggesting an additional mechanism of action other than the proximal tubular natriuretic effect of isradipine in normotensive volunteers.