We conducted a prospective evaluation of the effects of chronic captopril therapy on glucose tolerance in 8 nondiabetic, hypertensive patients and 6 hypertensive patients with impaired glucose tolerance, including 3 diabetic patients. Captopril was well tolerated by all patients, and no untoward effects were observed. Chronic captopril therapy produced a significant decrease in blood pressure in all patients. No patients with normal glucose tolerance developed diabetes mellitus. Neither fasting nor post-glucose-load venous plasma glucose deteriorated in any patients during chronic captopril therapy. There were no significant changes in the insulinogenic index (delta IRI/delta BS at 30 min post-glucose load) in patients with either normal or impaired glucose tolerance. These results suggest that, in addition to its antihypertensive effect, chronic captopril therapy does not compromise glucose metabolism in hypertensive patients. Thus, captopril may have a clinical advantage in that it apparently can be given safely to hypertensive patients with either normal or impaired glucose metabolism.