Adult female rats were subjected to an eleven-week endurance-training programme, and, for the first time, the maximum activities of enzymes that can indicate the quantitative capacities of both anaerobic glycolysis and the Krebs cycle in muscle (viz. 6-phosphofructokinase and oxoglutarate dehydrogenase respectively) were measured in heart plus white and fast-oxidative skeletal muscle. No changes were observed in heart muscle. In fast-oxidative skeletal muscle, activities of hexokinase, citrate synthase, and oxoglutarate dehydrogenase were increased by 51, 26, and 33% respectively but there was no effect on 6-phosphofructokinase. These results demonstrate that in red muscle there is no effect of this training programme on the anaerobic capacity but that of the aerobic system is increased by one third. In white skeletal muscle, only the activity of citrate synthase was increased, which indicates that this activity may not provide even qualitative information about changes in capacity of the Krebs cycle.