The role of a dual angiotensin (Ang) II-forming pathway from the local renin angiotensin system (RAS) of the cardiac tissue was determined in a hamster model of cardiac hypertrophy. Time-dependent expressions of chymase and angiotensin converting enzyme (ACE) genes and their enzymes activities, and Ang II levels were measured in the hamster heart at 3 days, and at 4 and 8 weeks after pressure overload, Cardiac hypertrophy was induced by an operation to constrict the abdominal aorta. Compared to the sham-operated group, the cardiomyocyte diameters of hamster hearts at 3 days after overload underwent no obvious changes, while those at 4 and 8 weeks after overload increased markedly (p<0.01), and both transcriptional expressions of chymase and ACE genes gradually increased in the hamster hearts at 3 days, and at 4 and 8 weeks after overload, but the transcriptional expressions of angiotensin II type 1 receptor (AT1R) gene gradually decreased. Chymase and ACE activities (U/mg) (0.441 +/- 0.040 vs. 0.175 +/- 0.014, 0.446 +/- 0.036 vs. 0.160 +/- 0.016 and 0.522 +/- 0.014 vs. 0.148 +/- 0.038) (p<0.01) and (0.142 +/- 0.023 vs. 0.056 +/- 0.038, 0.317 +/- 0.017 vs. 0.079 +/- 0.016 and 0.466 +/- 0.010 vs. 0.098 +/- 0.003) (p<0.01), respectively and Ang II levels (pg/g) (98.7 +/- 4.5 vs. 71.2 +/- 4.9, 134.4 +/- 7.8 vs. 71.9 +/- 12.8 and 151.6 +/- 10.1 vs. 80.7 +/- 3.0) gradually increased in the hamster hearts, vs. sham treatment, respectively, at 3 days, and at 4 and 8 weeks after overload. However, the increases in chymase and ACE activities were much higher than those in their respective mRNA levels, and the levels of chymase activities were also higher than those of ACE activities during the development of cardiac hypertrophy. The results suggested that the increase in Ang II levels via the dual pathway of Ang II formation by chymase and ACE plays an important role in the cardiac hypertrophy of hamsters caused by the overloaded state. Importantly, in the non-hypertrophied hamster heart in the early stage after overload (at 3 days), chymase could be activated by mechanical stress in advance of an increase in its mRNA, and the Ang II level increased significantly.