Human chymase forms angiotenin (ANG) I to ANG II, whereas the roles of ANG II generated by chymase and the effects of chymase inhibitors have been unclear. On the other hand, rat chymase could not convert ANG I to ANG II. In isolated rat arteries, the ANG I-induced vascular contraction was completely suppressed by angiotensin-converting enzyme (ACE) inhibitor only. However, 30% of ANG I-induced vascular contraction in isolated human arteries was suppressed by an ACE inhibitor, but the remainder was blocked by chymostatin. In hamster hypertensive models, ANG II formation by ACE, but not by chymase, in vascular tissues plays an important role in maintaining hypertension. ANG II formation also induces vascular remodeling such as neointima formation. After balloon injury of vessels in dog, chymase and ACE activities were significantly increased in the injured vessels. In this model, an ANG II receptor antagonist was effective in preventing neointimal formation after balloon injury of vessels in dog, but an ACE inhibitor was ineffective. In dog grafted veins, the activities of chymase and ACEmin the grafted vein were significantly increased 15- and 2-fold, respectively, compared with those in the symmetrical veins. The intimal area of the grafted vein was reduced by a chymase inhibitor. Therefore, chymase-dependent ANG II formation plays an important role in the proliferative response, and chymase inhibitors may appear useful for preventing vascular proliferation.