Studies were undertaken, using the isolated perfused rat heart, into various aspects of anoxia-induced enzyme release from the potassium arrested myocardium. Profiles for release, over periods of up to 8 h anoxia, were obtained for protein, creatine phosphokinase, myokinase, glutamate oxaloacetate transaminase, glyceraldehyde-3-phosphate dehydrogenase and α-hydroxybutyrate dehydrogenase. Two distinct peaks of enzyme release were observed; phase 1 release, which occurred during the first 10 to 60 min of anoxia and phase 2 release which followed phase 1 release and lasted for at least 7 to 8 h. In phase 1, 2 to 5% of the enzyme was released. Reoxygenation of the heart during phase 1 had a marginal effect upon overall enzyme release. Reoxygenation during phase 2 of enzyme release resulted in an immediate and massive increase in the rate of protein and enzyme release from the myocardium.