Abstract CW-1 mice of various ages (6, 22, and 27 months old) were exercised for up to 30 min daily on a treadmill for 5 weeks. At the end of every week hearts from six animals were removed, weighed, and homogenized. Enzyme activity levels of aldolase, superoxide-dismutase, and creatine kinase were measured as a function of age and training. Heart muscle weights increased slightly as a function of animal age but were not altered significantly by 5 weeks of training. Also, DNA and soluble protein levels in cardiac muscle did not change in 5 weeks of training. Activity levels of aldolase, superoxide-dismutase, and creatine kinase, however, increased consistently in young animals (6 months), whereas in old animals (27 months) no such positive adaptation was observed. In some instances, aldolase and SOD activities in old animals were reduced by as much as 50% after 5 weeks of training.