Indolapril (CI-907) is a new orally active prodrug of nonsulfhydryl angiotensin-converting enzyme (ACE) inhibitor, developed by Warner-Lambert/Parke-Davis Pharmaceutical Research for treating hypertension. Indolapril is epimer of trandolapril, well-known ACE inhibitor currently in the market for hypertension treatment. Indolapril (Monoester form) and it’s active component (diacid form) produced concentration related ACE inhibition in guinea-pig serum (IC50 for monoester -- 0.1 mkM and for diacid -- 2.6 nM). In isolated rabbit aortic rings and in rat and dog autonomic studies, Indolapril is highly specific in suppressing the contractile or pressor responses to angiotensin I. In two-kidney, one-clip Goldblatt hypertensive rats, single daily doses (0.03-30 mg/kg p.o.) produced dose-dependent decreases in blood pressure; 3 mg/kg lowered blood pressure to normotensive levels. In the spontaneously hypertensive rat, subacute administration of Indolapril produced the same decrease in blood pressure as that obtained in the renal hypertensive rat. In diuretic-pretreated renal hypertensive dogs, 10 mg/kg normalized blood pressure. For equivalent drops in blood pressure, heart rate increases were less in Indolapril than in enalapril-treated renal hypertensive dogs. No side effects were observed with CI-907 in any of the conscious animals. The antihypertensive response to Indolapril (0.03-1.0 mg/kg p.o.) was found to correlate with inhibition of vascular tissue ACE, but not plasma or brain ACE in two-kidney, one-clip renal hypertensive rats.