Thirty-nine hypertensive patients were entered into a randomized, double-blind protocol to assess the effects of enalapril (10–20 mg b.i.d.), or combined enalapril (10–20 mg b.i.d.) and hydrochlorothiazide (25–50 mg b.i.d.) therapy on renal function and hemodynamics. Enalapril, either alone or in combination with hydrochlorothiazide, effectively controlled blood pressure. In patients with an initial inulin clearance ≤ 80 ml/min/1.73 m<sup>2</sup>, inulin and p-aminohippurate clearances were markedly improved toward normal following either drug therapy. The filtration fraction was either unchanged (monotherapy) or increased (combination therapy), suggesting a direct glomerular effect of angiotensin-converting enzyme inhibition. Renal vascular resistance was decreased in all patients. These observations suggest that enalapril, either alone or in combination with a diuretic, has the potential to reverse renal function abnormalities encountered in the hypertensive state.