The effects of acute (single-dose) and chronic (26-day) therapy with nitrendipine on renal function were evaluated in 10 patients with mild to moderate hypertension. The patients were studied during three phases: for one week while on a constant diet containing 100 mEq of sodium and 80 mEq of potassium daily and receiving placebo, after administration of a single 20-mg oral dose of nitrendipine, and during one week of treatment with nitrendipine at 10 mg twice daily. After two weeks of outpatient treatment at 10 or 20 mg twice daily, the patients were readmitted and reevaluated while on controlled diets. Isotopic determinations of glomerular filtration rate, effective renal blood flow, blood volume, and cardiac output were made during each phase, and free water clearance and osmolar clearance following a water load were measured; during the periods of hospitalization, 24-hour urinary creatinine and electrolyte excretion were assessed. A significant decrease in blood pressure occurred during both the acute and chronic phases as compared with the placebo phase. During the first week of treatment with nitrendipine, a significant increase in urinary sodium excretion over control values was observed, with a mean deficit of 148 +/- 7 mEq (p less than 0.001); a mean weight loss of 1.0 +/- 0.1 kg during this period was also significant (p less than 0.05). No substantial changes in glomerular filtration rate, renal blood flow, blood volume, cardiac output, plasma renin activity, and levels of plasma catecholamines or urinary aldosterone from control values were noted during either the acute or chronic phases.(ABSTRACT TRUNCATED AT 250 WORDS)