In vascular tissues, angiotensin II is potentially cleaved from angiotensin I by chymase and angiotensin converting enzyme (ACE). In the normal state, ACE regulates angiotensin II formation and plays a crucial role in the regulation of blood pressure, whereas chymase is stored in mast cells and has no angiotensin II-forming activity. Chymase is activated immediately upon its release into the extracellular matrix in vascular tissues after mast cells have been activated by local stimuli such as vessel injury by grafting or a balloon catheter. In dog grafted veins, vascular proliferation, chymase activity, angiotensin II concentration and mRNA levels of fibronectin, collagen I and collagen III were significantly increased after the operation, while they were significantly suppressed by a chymase inhibitor. A clinical trial of an angiotensin II receptor blocker (ARB) for preventing restenosis after percutaneous transluminal coronary angioplasty was successful, but that of an ACE inhibitor was not. After balloon injury in dog vessels, chymase activity was signifcantly increased in the injured artery, and a chymase inhibitor and an ARB were effective in preventing the vascular proliferation, but an ACE inhibitor was ineffective. On the other hand, a chymase inhibitor, unlike an ACE inhibitor and an ARB, did not affect blood pressure. These reports indicate that local angiotensin II production by chymase is involved only in the intimal hyperplasia seen in the injured vessels. Therefore, chymase inhibitors may be useful for preventing vascular disoders without affecting blood pressure.