The activities of hexokinase (HK), lactate dehydrogenase (LDH), citrate synthase (CS) and 3-hydroxyacyl-CoA-dehydrogenase (HAD) were measured in the costal diaphragm (DI) and in the latissimus dorsi (LD), a non respiratory muscle, of 21 middle aged men: 10 normal subjects and 11 patients with moderate chronic obstructive pulmonary disease (COPD). In the normal subjects, all the enzymatic activities were significantly higher in the diaphragm (p less than 0.01) than in the latissimus dorsi, the mean increase ranging from 61 to 118%. Chronic obstructive pulmonary disease failed to cause any further increase in enzymatic activities in the DI; on the contrary, a significant decrease of HK and LDH (30%, p less than 0.01) was observed. Enzymatic activities of LD were unaffected by COPD. These data show that the metabolic enzymatic activities in the diaphragm of normal middle aged subjects is similar to that of the vastus lateralis of highly trained young athletes, suggesting that DI is a highly trained muscle in normal man.