The acute hypotensive and renal effects of the calcium antagonist tiapamil, a verapamil derivative, were studied in nine normal and 13 essential hypertensive subjects undergoing water diuresis. Tiapamil decreased mean arterial pressure slightly in normotensive subjects and more markedly in hypertensive patients (-6 +/- 1 versus -10 +/- 2%). The effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were unaffected in both groups. A striking natriuresis was observed selectively in the hypertensive group (+344 +/- 84 versus +42 +/- 22 mmol/min) and was associated with an increase in the calculated fractional distal delivery of sodium and uric acid excretion rate. Plasma aldosterone concentration decreased moderately and to the same extent in both groups. In conclusion, tiapamil induced a marked natriuresis in essential hypertensive patients, despite a decrease in blood pressure, and in the absence of renal vasodilatation; this may suggest the existence in essential hypertension of a calcium-linked abnormality in the renal proximal tubular handling of sodium.