31-Phosphorus magnetic resonance spectroscopy was used to investigate in vivo the kinetics of inorganic phosphate transport and intracellular pH after exercise in human skeletal muscle. Intracellular pH further decreased from the value reached at the end of work showing a minimum between 25 and 45 sec and then increased back to the resting value. Inorganic phosphate showed an initial fast rate of recovery corresponding to the decreasing phase of pH, and a second phase in which a slow rate of recovery corresponded to increasing pH. The biphasic patterns of both phosphate and pH recoveries are in agreement with and support in vitro evidence that Pi transport into mitochondria is modulated by pH.