Chymase shows a catalytic efficiency in the formation of angiotensin (Ang) II. In the present study, the characterization and primary structure of monkey chymase were determined, and the pathophysiological role of chymase was investigated on the atherosclerotic monkey aorta. Monkey chymase was purified from cheek pouch vascular tissue using heparin affinity and gel filtration columns. The enzyme rapidly converted Ang I to Ang II (K_m=98 μM, k_cat=6203/min) but did not degrade several peptide hormones such as Ang II, substance P, vasoactive intestinal peptide and bradykinin. The primary structure, which was deduced from monkey chymase cDNA, showed a high homology to that of human chymase (98%). The mRNA levels of the aorta chymase were significantly increased in the atherosclerotic aorta of monkeys fed a high-cholesterol diet. These results indicate that monkey chymase has a highly specific Ang II-forming activity and may be related to the pathogenesis of atherosclerosis.