High resolution proton magnetic resonance spectra of intact frog muscle, obtained by the selective saturation of water signal with 15 min accumulation, showed well-resolved signals from lactate, carnosine, and other compounds. This technique was used to monitor the changes in lactate content and in intracellular pH which was estimated from the chemical shift of carnosine in muscle. The intracellular pH was estimated to be 7.1 in fresh muscles. Under an anaerobic condition, resting muscle showed a gradual increase in lactate content with intracellular acidification. When the anaerobic muscle was electrically stimulated for 5 sec, a transient acceleration of the lactate production and the intracellular acidification was observed. The intracellular buffer value was estimated to be 27 mEq/(pH X kg muscle), based on the pH change due to the increase in lactate content.