The systemic and renal responses to normal volume expansion (1800 ml isotonic saline infused over 3 h) were assessed in 23 patients with essential hypertension after 4-7 days' administration of a placebo or the calcium antagonist nitrendipine (20 mg three times a day) or the angiotensin converting enzyme (ACE) inhibitor captopril (50 pressure mg three times a day). Each drug treatment was associated with a fall in blood pressure of the same magnitude. Treatment with captopril was associated with a significant volume dependence of blood pressure and a blunting of the exaggerated natriuresis of hypertension (71 +/- 16 on placebo and 47 +/- 12 mmol/3 h on captopril; P less than 0.01). Treatment with nitrendipine did not affect the change in blood pressure associated with volume expansion, but significantly enhanced the natriuretic response to volume expansion (61 +/- 8 on placebo and 90 +/- 16 mmol/3 h on nitrendipine; P less than 0.01). No alterations in baseline or volume expansion-induced changes in the glomerular filtration rate or in plasma renin activity were observed during treatment with nitrendipine. Therefore, the exaggerated natriuresis of essential hypertension was blunted by the ACE inhibitor and enhanced by the calcium antagonist, despite a similar reduction in pre-expansion arterial pressure.