A number of serum enzymes increase after strenuous exercise. However, there is no information concerning these enzymes after routine treadmill testing. SGOT, CPK and LDH were determined before, immediately after, at 4 and 24 h postexercise in 100 subjects undergoing this test. When the group was considered as a whole, only the SGOT showed a slight but statistically significant elevation immediately after exercise. The mean values of the SGOT at 4 and 24 h, as well as the mean values for the CPK and LDH at all times after exercise, were not significantly increased. SGOT increased more significantly immediately after exercise in those patients who exercised more than 6 min. This group of patients also showed modest but statistically significant elevations of their CPK immediately after completion of the exercise test. Analyses of the data in relation to the presence or absence of ischemic ST shifts and presence or absence of documented coronary obstruction, failed to show any significant correlation. The effect of conditioning on the serum enzyme changes postexercise was not demonstrable.