The feline cardiac and serum angiotensin converting enzyme (ACE) and chymase activities were determined and compered in dogs, and hamsters. In all three species, cardiac chymase activity exceeded ACE activity; however, there were some differences. In cats, left ventricular ACE and chymase activities (0.15 ± 0.01 and 0.59 ± 0.1 mU/mg-protein, respectively) were lower than in dogs (0.42 ± 0.05: p<0.01 and 2.0 ± 0.4 mU/mg-protein: p<0.01) and hamsters (0.93 ± 0.06: p<0.001 and 2.1 ± 0.2 mU/mg-protein: p<0.01); in contrast, serum ACE activities was higher in cats (12.7 ± 1.0 mU/ml) than in dogs (5.9 ± 0.6 mU/ml: p<0.001). The relative contribution of chymase (cats: 84.0 ± 5.1%, dogs: 81.4 ± 3.4%, and hamsters: 72.6 ± 5.6 %) to ANG-II formation in the heart was greater than that of ACE in these animals (cats: 10.9 ± 4.1%, dogs: 11.5 ± 3.6%, and hamsters: 17.2 ± 0.8%). These species-specific differences suggest that the efficacy of renin-angiotensin system modulating agents may differ among species.