The catalytic activities of several oxidative and glycolytic enzymes were determined in the gastrocnemius muscle of 10 mammalian species differing in body weight by nearly 6 orders of magnitude. When expressed in terms of units gm-1, the activities of enzymes functioning in oxidative metabolism (citrate synthase, beta-hydroxybutyrylCoA dehydrogenase, and malate dehydrogenase) decrease as body weight increases. Log-log plots (activity gm-1 vs body mass) yield straight lines with negative slopes that are less than the allometric exponent (-0.25) typically observed for basal metabolic rates. Since the amount of power a muscle can generate depends upon the catalytic potential of its enzyme machinery (the higher the catalytic potential the higher the maximum rate of energy generation), these data predict that the scope for aerobic activity in large mammals should be greater than in small mammals if nothing else becomes limiting, a result in fact recently obtained by Taylor et al. (Respir. Physiol., 1981). In contrast to the scaling of oxidative enzymes, the activities of enzymes functioning in anaerobic glycogenolysis (glycogen phosphorylase, pyruvate kinase, and lactate dehydrogenase) increase as body size increases. Log-log plots (activity gm-1 vs body mass) display a positive slope indicating that the larger the animal the higher the glycolytic potential of its skeletal muscles. This unexpected result may indicate higher relative power costs for burst type locomotion in larger mammals, which is in fact observed in within-species studies of man. However, the scaling of anaerobic muscle power has not been closely assessed in between-species comparisons of mammals varying greatly in body size.