The effects of felodipine on renal function were evaluated in dogs and rats using renal clearance, haemodynamic and micropuncture techniques. Unilateral renal arterial infusion of felodipine in anaesthetised dogs produced an ipsilateral diuresis and natriuresis without changing the mean arterial blood pressure, renal blood flow or glomerular filtration rate. In similar studies, the non-calcium antagonist vasodilator, hydralazine, produced antidiuresis and antinatriuresis. Continuous intravenous administration of felodipine in anaesthetised normotensive rats also produced a diuresis and natriuresis without changing mean arterial blood pressure, renal blood flow, whole kidney or single nephron glomerular filtration rate. Renal tubular reabsorption of water and sodium were inhibited in the distal tubule and collecting duct; neither potassium excretion nor reabsorption were affected. Bolus intravenous administration of felodipine to conscious spontaneously hypertensive rats produced dose-related decreases in mean arterial blood pressure and renal vascular resistance which were associated with increases in renal blood flow, glomerular filtration rate, urinary flow rate and sodium excretion; urinary potassium excretion was not affected. The non-calcium antagonist vasodilator minoxidil produced antidiuresis and antinatriuresis. Felodipine, a calcium-antagonist vasodilator used as an antihypertensive, possesses beneficial diuretic and natriuretic properties which are due to its direct effect on renal tubular water and sodium reabsorption.