To assess the rates of change in muscle metabolites such as phosphocreatine (PCr) and inorganic phosphate (Pi) during repeated exercise sessions with rest periods, 31-phosphorus nuclear magnetic resonance spectroscopy was used for continuous and noninvasive measurements. Five long-distance runners and six healthy male subjects as controls performed a 2-min femoral flexion exercise at 20 kg.m.min-1 in a 2.1 T superconducting magnet with a 67-cm bore; they repeated this exercise four times with a 2-min rest period. At the beginning of exercise, PCr decreased exponentially; at the end, it increased. During exercise and in the early phase of the recovery in every exercise session, the PCr values were significantly higher in the long-distance runners than in the control subjects (P < 0.05). The Pi increases and decreases involved with exercise also revealed exponential changes. The Pi values did not significantly differ during exercise; however, Pi recovery was faster in the long-distance runners than in the control subjects (P < 0.05). The Pi:PCr ratio during exercise increased linearly with exercise; and Pi:PCr during recovery was smaller in the long-distance runners than in the control subjects (P < 0.05). In conclusion, the long-distance runners revealed faster PCr and Pi kinetics after exercise and a smaller Pi:PCr during exercise than did the control subjects. It is suggested that these results were attributable to a greater oxidative capacity of muscles in the long-distance runners.