Abstract: MK-421, a new angiotensin-Ⅰ converting enzyme inhibitor, was found to reduce the blood pressure in one-kidney, one-clip hypertensive rats at 30 mg/kg po (n=12). On the 5 th day and the 5 th week after renal artery constriction the blood pressure decreased from 196±14 and 250±5 mmHg pretreatment to 159±28 and 225±3 mmHg respectively (p<0.001 and 0.05 respectively) and maintained the level about 8 hours. After treatment with MK-421,angiotensin-Ⅰ converting enzyme activity was significantly inhibited. Similarly, MK-421 (30 mg/kg po) also decreased the blood pressure in two-kidney, one-clip hypertensive rats (n=18) on the 5th day and the 5th week after operation. The blood pressure decreased from 162±20 and 217±20 mmHg to 117±24 and 120±28 mmHg respectively and reduced to preoperation normortensive level(113±5 mmHg). These results indicate that renin-angiotensin systemmightplay an important role in the pathogenesis of two-kidney one-clip hypertensive rats. However, besides renin-angiotensive system, there are other mechanisms involved in the pathogenesis of one-kidney,one-clip hypertensive rats.