Phosphorus nuclear magnetic resonance spectra at 145 MHz were obtained with a surface coil from the gastrocnemius-plantaris muscles of anesthetized rats. Phosphocreatine (PCr) and inorganic phosphate (Pi) contents and intracellular pH were measured before, during, and after periods of contractile activity induced by twitch or tetanic stimulation of the sciatic nerve. Reduced levels of PCr, increased levels of Pi, and intracellular acidification were achieved in a graded fashion with increased rates of twitch stimulation from 2 to 10 Hz without detectable changes in the ATP content. In all cases, the decrease in PCr was matched by a stoichiometric increase in Pi content. The time constant of resynthesis of PCr averaged 30 s, was five times faster than the restoration of intracellular pH to control levels, and was independent of the degree of intracellular acidosis at the beginning of recovery.