In order to evaluate the effect of exercise on serum creatine kinase levels, blood samples were obtained from 17 normal females and 12 Duchenne muscular dystrophy carriers before and 9 hours after moderately strenuous exercise. The results revealed that after exercise serum creatine kinase levels may be better indicators of carrier status than resting levels. The mean serum creatine kinase levels before and after exercise, as well as the mean increases, were found to be significantly greater in Duchenne muscular dystrophy carriers than in normal control subjects.