Thirty-eight direct electrocardiographic records have been obtained by applying an exploring electrode directly to the epicardium after incision of the pericardium in the case of seven patients subjected to thoracotomy for various forms of heart surgery. These cases include three individuals with the tetralogy of Fallot, one with mitral stenosis, one with coarctation of the aorta, one with probable pulmonary atresia and one with a patent ductus arteriosus. This exploration has permitted us to study the patterns obtained in different regions of the right and left ventricular epicardial surfaces in four patients with right ventricular hypertrophy, two patients with left ventricular hypertrophy, and one patient with a heart whose left ventricle-right ventricle ratio was probably little disturbed.