The release of lactate, phosphate and purine catabolites from the heart in adult and children undergoing cardiac surgery was recorded. The compounds were determined in the coronary effluent collected during subsequent infusions of cardioplegic solution into the coronary root. As compared to the infusion just after onset of ischemia, both in adults and children manifold increase of the release was observed during subsequent infusions. The rates of release of lactate, phosphate and purines (adenosine + inosine + hypoxanthine) were 1.5 to 2.5 times higher in children than in adult hearts during the second cardioplegic infusion and 3 to 7 times higher during the third cardioplegic infusion in spite of a more frequent infusion of cardioplegic solution in children. A much greater increase of the release of lactate, phosphate and purines provides evidence for more severe metabolic injury during cardioplegic arrest to the heart in children than in adults.