A graded multistage treadmill exercise test was performed on 78 patients with myocardial infarction (65 males and 13 females, ranging in age from 30 to 76). The amplitude of the R wave in CC5 was measured at rest and during the periods of peak exercise. The exercise-induced R wave changes were classified into 3 groups: increased, 39.1%; no changes, 39.7% and decreased, 22.2%. Acute attacks of myocardial infarction were more severe in the patients whose R wave decreased during exercise than in those whose R wave increased. Heart rate, blood pressure and pressure rate products were not different among the groups. However, oxygen consumption was greater in the R-increased group than in the R-decreased group. Therefore, an increase in R wave amplitude during exercise in post-myocardial infarction patients indicates a good efficiency of the myocardium and skeletal muscles. Exercise-induced changes of the mean electrical axes of the transverse plane were significantly related to changes of the R wave amplitude in CC5 (r = 0.50, p less than 0.001). Therefore, change in the transverse plane axis is one of the important determinants of exercise-induced R wave changes.