Hypertension with diabetes mellitus has been associated with suppression of the renin-angiotensin-aldosterone system. We have studied the effects of the converting enzyme inhibitor, captopril, on blood pressure, plasma renin activity (PRA) and plasma aldosterone in 10 stable hypertensive diabetic subjects and 10 age-matched patients with essential hypertension. There was no clinical evidence of complication in the diabetic subjects and their diabetic treatment remained unchanged throughout the study. Mean captopril doses used were similar in both groups. In the diabetics and the essential hypertensives, treatment resulted in a significant and similar decrease in blood pressure. Pre-treatment basal and stimulated PRA and the change of PRA with captopril were also similar. Pre-treatment stimulated plasma aldosterone and the response of aldosterone to postural stress was significantly lower in the diabetic group, suggesting an impaired adrenal responsiveness to stress. Despite this, our findings indicate that the hypotensive action of captopril is at least as effective in hypertension associated with otherwise uncomplicated diabetes mellitus as in essential hypertension.