The antihypertensive efficacy of enalapril and its effects on the metabolism and kidney function were investigated in 11 insulin-dependent diabetic subjects with uncomplicated mild to moderate hypertension. During a short-term single-blind controlled trial, one daily dose of 20 or 40 mg enalapril significantly reduced both systolic and diastolic blood pressure. In the supine position, mean systolic blood pressure declined from 169 +/- 6 to 142 +/- 6 mmHg (P less than .01) and mean diastolic blood pressure from 101 +/- 1.5 to 85 +/- 2 mmHg (P less than .001). No changes in heart rate or postural hypotension were observed. During 1 yr of treatment, the antihypertensive efficacy of enalapril did not decline, and no clinical side effects were observed. Inhibition by enalapril of angiotensin-converting enzyme did not modify daily insulin requirements, glycemic control, uricemia, or lipid metabolism; kalemia and the markers of diabetic nephropathy were not significantly altered. These results suggest that enalapril once daily should be used as the first step in the treatment of diabetic patients with mild to moderate hypertension.