We have studied samples of myocardium from 40 patients with congenital heart disease for catecholamine, histamine, and glycogen content. Blood samples obtained before, during, and after cardiopulmonary bypass in 53 patients were assayed for total protein, osmolality, catecholamines, histamine, lactic acid dehydrogenase activity, and cholinesterase activity. The data show that (1) adequate cardiopulmonary bypass is accompanied by increased sympathetic activity, (2) myocardial damage, indicated by elevated plasma lactic acid dehydrogenase activity, may occur during operation, and (3) depletion of myocardial catecholamine content may be a concomitant of heart disease with chronic congestive failure.