In the present study we investigated the effect of a single dose, and 3 months of treatment with spirapril on kidney function, renin-angiotensin system, renal handling of sodium and blood pressure, in patients with reduced kidney function (serum creatinine 1.5-3 mg%) and hypertension. A single dose of 6 mg spirapril given at the beginning of the study did not affect glomerular filtration rate (GFR), renal plasma flow (RPF), angiotensin converting enzyme (ACE) activity, plasma renin activity (PRA) or renal handling of sodium. When the single dose of spirapril was given after 3 months of treatment with this agent, renal hemodynamics and PRA did not change. ACE activity, which was depressed by the previous spirapril treatment, decreased further (from 9.5 +/- 3.1 to 1.4 +/- 1.0 nmol/ml/min), (p < 0.05). Administration of 6 mg spirapril o.d. for 3 months did not have any effect on GFR or RPF. Serum ACE activity decreased from 92.1 +/- 8.0 to 5.1 +/- 2.6 nmol/ml/min (p < 0.05) and PRA increased from 1.4 +/- 1.2 to 4.1 +/- 3.6 ng/ml/min (p < 0.05). Plasma aldosterone did not change. Similar results were obtained when spirapril was combined with 5 mg isradipine in the initial and final single dose, or in the 3 months' treatment (5 mg o.d.). Blood pressure was normalized in 38% of the patients who received spirapril and in 71% of the patients who received spirapril and isradipine. Thus, (a) treatment with spirapril in patients with mild to moderate chronic renal insufficiency was not associated with deleterious effects on kidney function; (b) spirapril in a dose of 6 mg alone or in combination with 5 mg isradipine is effective in reducing blood pressure in hypertensive patients with reduced kidney function.