1. The rate of efflux of lactate from isolated frog sartorius muscles is measured with a superfusion technique. Efflux curves are followed after raising the internal lactate level of the muscles by repetitive electrical stimulation over a 200 sec period.2. With an external pH of 7.0 or below the measured efflux rates following stimulation reach 100-150 n-mole/g.min. Increasing the pH of the superfusion fluid to 8.0 results in a two or threefold increase in the peak efflux rate. The effect is independent of the buffer system used and occurs fairly rapidly when the pH of the superfusion fluid is changed. This suggests that the effect of pH on lactate efflux is extracellular.3. The increase in efflux rate due to an increase in pH is dependent on buffer concentration. This fact together with measurements of surface pH changes in muscles following arrest of superfusion indicates that a pH gradient exists through the muscle thickness during lactate efflux.4. The low lactate efflux rate seen at a low buffer concentration (1 mM) is reduced to an even lower level by depolarization with potassium sulphate suggesting a membrane potential dependent component. At pH 8.0 with a high buffer concentration (25 mM) potassium sulphate only reduces efflux rate slightly.The observations are interpreted as indicating that a fraction of lactate lost is in the form of undissociated acid and that this fraction increases with increasing external pH.5. Conditions which favour loss of hydrogen ions and lactate from muscle are also associated with improved recovery of twitch tension.