Cardiac creatine phosphokinase (CPK) activity in international units (IU) was normalized to homogenate soluble protein (IU/mg), to tissue wet weight (IU/100 mg) or to tissue dry weight (IU/mg). Samples of cardiac tissue were excised from sham-operated cats and dogs or from cats after 4 h of severe pericardial tamponade and dogs after 4 hr of cirumflex artery ligation. In sham-operated cats and dogs, cardiac CPK activity was about 30 IU/mg homogenate soluble protein, 200 IU/100 mg wet tissue, and 9.5 IU/mg dry tissue. In cats, after tamponade, the CPK activity of subepicardial and subendocardial tissue of the left ventricle was decreased 24-35% if the enzyme activity was referenced to wet weight or dry weight. Similarly, in dogs, after circumflex artery ligation, the CPK activity of posterior papillary muscles was reduced 24-30% if the enzyme activity was referenced to tissue wet or dry weight. In both cats subjected to 4 h of tamponade and dogs after 4 h of circumflex artery ligation, significant loss of CPK activity could not be demonstrated if the cardiac enzyme activity was normalized only to the soluble protein content of homogenates. We conclude that normalizing cardiac CPK activity to tissue wet and dry weight may permit early assessment of myocardial injury in experimental models of myocardial ischemia.