Sarcolemmal giant vesicles obtained from rat hindlimb muscles were used as a model for the study of pH regulation in skeletal muscle. The transport systems involved in the recovery from 40 mM lactate and pH i 6.5 were quantified from both flux measurements of the co-transported ions and counter-ions, and from measurements of the rate of the internal pH change. The diffusion of lactic acid plus the carrier-mediated co-transport of lactate and H + had the highest capacity to transport protons (240 nmol H + /mg protein per min). These systems are therefore responsible for a large part of the H + efflux in periods with a high lactate production. The capacity of the HCO - 3 -dependent systems was 47 nmol/mg per min, and the capacity of the Na + /H + exchange system was 33 nmol/mg per min in vesicles from mixed muscles. The capacity to remove H + by the lactate/H + co-transport system and by the bicarbonate-dependent systems was significantly higher in vesicles from predominantly red fibers than in vesicles from white fibers, whereas the distribution of the Na + /H + exchange system was independent of fiber type. These observations demonstrate that the pH regulation during muscle activity in red muscles is more effective than in white muscles.