The metabolic state of human muscle in various functional states has been investigated by the non-invasive technique of 31P nuclear magnetic resonance. The concentrations of phosphocreatine, ATP and inorganic phosphate as well as intracellular pH in the flexor digitorum superficialis have been measured during rest, dynamic exercise and recovery from exercise. The observed relationship between phosphocreatine utilization and decrease in intracellular pH during aerobic exercise indicates that lactate production only becomes significant after more than 60% of the phosphocreatine is used up. Surprisingly intracellular pH may reach as low a value as 5.9 to 6.1, indicating that phosphofructokinase is still partially active at pH 6.0. There is no metabolic recovery if the muscle is made ischaemic following exercise, implying that glycolysis is switched off as soon as exercise is stopped. Lactic acidosis is not the cause of this and presumably Ca2+ is needed to maintain the activation of phosphorylase kinase. The time-course of phosphocreatine recovery after exercise reflects the rate of oxidative metabolism, while pH recovery probably represents H+ ion export from the muscle cell. The dynamics of metabolic changes can now be observed with a time resolution of 10 to 60 seconds and thus disturbances in energy metabolism can be readily detected in several pathological states.