Effect of BR-900317 on the angiotensin I-induced pressor response in pithed rats and the effects of its single oral administration on plasma angiotensin converting enzyme (ACE) activities in normotensive rats and on the cardiovascular system in hypertensive model rats (SHR, RHR), were compared with those of captopril. BR-900317 attenuated the angiotensin I-induced pressor effects in pithed rats. In a single oral dose administration study, BR-900317 inhibited the plasma ACE activities in a dose-dependent fashion. Duration of the action of BR-900317 was similar to that of captopril. BR-900317 produced antihypertensive effect in spontaneously hypertensive rats and dose-dependent antihypertensive effect in 2-kidney Goldblatt hypertensive rats without affecting heart rate. These results suggest that the main mechanism of the antihypertensive effect of BR-900317 is the suppression of angiotensin II production due to the inhibition of the ACE.