Exercise is associated with a net loss of K+ from the working muscles and an increased plasma K+ concentration, indicating that the capacity for intracellular reaccumulation of K+ is exceeded. Training reduces the exercise-induced rise in plasma K+, and an increased plasma [K+] may interfere with physical performance. Since the clearing of K+ from the extracellular space depends on the capacity for active K+ uptake in skeletal muscle, the effects of training and inactivity on the total concentration of (Na+ + K+)-ATPase was determined. Following 6 weeks of swim training, the concentration of [3H]ouabain-binding sites in rat hindlimb muscles was up to 46% (P less than 0.001) higher than in those obtained from age-matched controls. Whereas muscle Na+, K+ contents remained unchanged, the concentration of citrate synthase increased by up to 76% (P less than 0.001). Training induced no change in the [3H]ouabain-binding-site concentration in the diaphragm, but in the heart ventricles, the K+-dependent 3-O-methylfluorescein phosphatase activity increased by 20% (P less than 0.001). Muscle inactivity induced by denervation, plaster immobilisation or tenotomy reduced the [3H]ouabain-binding-site concentration by 20-30% (P less than 0.02-0.001) within 1 week. In conclusion, training leads to a significant and reversible rise in the concentration of (Na+ + K+)-ATPase in muscle cells. This may be of importance for the beneficial effects on physical performance by improving the maximum capacity for K+ clearance.