Abstract It was previously found that the enzymes of the mitochondrial respiratory chain increase in leg muscles of rats subjected to a program of running. The results of the present study provide evidence that mitochondrial coupling factor 1 (F1) is increased in proportion to the constituents of the respiratory chain in muscle that has adapted to endurance exercise. The oligomycin-sensitive adenosine triphosphatase (ATPase) activity of the mitochondrial fraction of muscle, after disruption of the mitochondria and unmasking of the latent ATPase activity, was used as a measure of F1. The concentration of cytochrome c was used as a marker for the respiratory chain. Both mitochondrial ATPase and cytochrome c increased 2-fold in gastrocnemius muscle in response to a running program, suggesting that a coordinated, quantitatively related increase in these components of the mitochondrial cristae occurs during the adaptation to exercise. In contrast, the levels of activity of mitochondrial creatine phosphokinase and adenylate kinase did not increase, providing evidence of a change in mitochondrial composition. Cytoplasmic creatine phosphokinase and adenylate kinase activities were also unchanged. These findings reinforce previous evidence indicating that endurance exercise results in an adaptive increase in the capacity of muscle to regenerate ATP aerobically, whereas the ability to form ATP by anaerobic mechanisms is unaffected.