Objectives: To study the function of in vivo chymase on heart remodeling by overexpression of human chymase in the heart of transgenic mice. Methods: Transgenic mice were produced by microinjection. The chymase mRNA levels in the heart and other tissues were assessed by competitive reverse transcriptase-polymerase chain reaction (RT-PCR). The expression of collagen I/III genes was analyzed by Northern blot hybridization. Chymase and angiotensin-converting enzyme (ACE) activities, and angiotensin II (Ang II) content in the heart were determined by radioimmunoassay (RIA). The matrix metalloprotease-9 (MMP-9) in protein and activity levels were measured by Western blot and zymogram, respectively. Results: A model of transgenic mice with selective overexpression of a rat myosin light chain 2 promoter-human heart chymase (MLC2-hChymase) fusion gene was produced. In MLC2-hChymase transgenic mice (the F6 line), the human heart chymase gene was expressed at a high level in heart and at lower levels in skeletal muscle and kidney, while no expression was detected in the liver or lung. The heart chymase activity increased markedly in the F6 transgenic mice versus non-transgenic mice (0.274 ± 0.071 U/mg versus 0.152 ± 0.021 U/mg) (P<0.05), with no difference in ACE activity. Heart Ang II level in the F6 transgenic mice increased nearly threefold (1984 ± 184 versus 568 ± 88 pg/g protein) (P<0.05) but was unchanged in plasma. MMP-9 activity increased significantly in the cardiac tissue of F6 transgenic mice (P<0.05), while both collagen I and the ratio of collagen I: III mRNA levels decreased significantly (both P<0.05). The F6 transgenic mice showed no significant changes in cardiac parameters. Conclusions: We have demonstrated selective overexpression of human chymase gene in the heart of transgenic mice, and the results support the hypothesis of a dual Ang II-forming pathway from chymase and ACE in the cardiac tissue in vivo,. The results also suggest that chymase may play a role in heart remodeling by increasing Ang II formation and activating MMP-9, and the regulation of collagen I gene expression. © 2002 Lippincott Williams & Wilkins.