The effects of pH on the kinetics of fatigue and recovery in frog sartorius muscle were studied to establish whether the pH to which muscles are exposed (extracellular pH) has an effect on both the rate of fatigue development and recovery from fatigue. When frog sartorius muscles were stimulated with short tetanic stimuli at rates varying from 0.2 to 2.0 trains/s, a time- and frequency-dependent decrease in force development was observed, but extracellular pH had comparatively little effect. The recovery of tetanic force was dependent on the extracellular pH. This effect was characterized by a rapid recovery in force at pH 8.0 and an inhibition of recovery at pH 6.4 even when force decreased by only 25% during stimulation. Even when muscles were fatigued at pH 8.0 the rate of force recovery was still very small at pH 6.4. A model is proposed in which a step of the contraction cycle changes from a normal to a fatigued state. The rate of this transition is a function of the stimulation frequency and not pH. The reverse transition, from a fatigued to normal state is pH dependent; i.e., it is inhibited by H+. Measurements of resting and action potentials show that extracellular pH influences these parameters in the fatigue state, but there is no evidence that these changes are directly responsible for the pH-dependent step in the reversal of fatigue.