Nicotinamide adenine dinucleotides (NAD, NADH2, NADP, and NADPH2) levels decrease in myocardial dog tissue after the ligature of the coronary artery branch. The activity of a glycohydrolytic enzyme acting on NAD and releasing nicotinamide in an equivalent amount was of the same order of magnitude in infarcted tissue, irrespective of the time elapsed after the coronary artery occlusion, as it was in normal tissue. Most of the NAD contained in normal heart muscle was hydrolyzed as soon as the tissue was disrupted in a homogenizer, whereas no hydrolysis occurred when the whole fragment was incubated for 1 hour. The enzymatic activity was found mainly in a membranous fraction seperated at 17,000 x g by differential centrifugation. Acid phosphatase, K+ -activated phosphatase, and NA+-K+-ATPase specific activities were greater in this fraction. It is suggested that the structural disorganization of the heart elicited either in vitro or during the infarction process determines the conditions for a reaction between the enzyme which is localized in the membranes and the NAD which is mainly in the cytosol.