The purpose of this study was to determine tissue angiotensin I-converting enzyme (ACE) activity in aging hamsters with and without cardiomyopathy, and the factors that regulate its activity in vitro. We found that ACE activity was significantly increased in the heart and significantly decreased in the lung of aging hamsters with hereditary cardiomyopathy in comparison to age/genetically-matched controls (P < 0.05). Kidney and cheek pouch ACE activity was similar in both groups. Lisinopril inhibition curves of tissue ACE activity were similar in aging hamsters with and without cardiomyopathy. In both groups, tissue ACE activity was dependent on chloride anion concentration in the assay buffer. Substituting citrate for chloride abrogated, in part, this response. We conclude that cardiomyopathy is associated with significant changes in cardiac and lung ACE activity in aging hamsters in comparison to age/genetically-matched controls. However, regulation of tissue ACE activity in vitro is similar in both groups.