1. Changes in intracellular pH and intracellular anion levels were monitored in frog sartorius muscle fibres during exposure to extracellular L-lactate, using ion-sensitive microelectrodes. 2. Resting intracellular pH (pHi) in 20 mmol l-1 HEPES buffer was 7.18 +/- 0.015 (S.E. of mean, n = 62). Exposure to an extracellular solution at pH 6.5 buffered with 20 mmol l-1 3-(N-morpholino)propanesulphonic acid (MOPS) resulted in a slow intracellular acidification. 3. A reversible decrease in pHi and an increase in intracellular anion levels was observed when L-lactate replaced chloride in equimolar amounts. The increase in intracellular anion level is consistent with intracellular accumulation of L-lactate ion. 4. The rate and steady-state change in pHi and anion level was a function of both extracellular pH and L-lactate concentration, providing evidence for the coupled movement of lactate and proton equivalents. 5. The initial rate of uptake of L-lactate, as measured by the change of pHi, was a non-linear function of the extracellular L-lactate concentration at extracellular pH 6.8 and 7.35. 6. No saturation was observed with concentrations of L-lactate between 5 and 60 mmol l-1 at pH 7.35 and 2.5 and 40 mmol l-1 at pH 6.8. 7. The non-linear relationship between the initial rate of change in pHi and extracellular L-lactate was well fitted by a curve defining uptake as the sum of a carrier process displaying Michaelis-Menten kinetics and a passive diffusion component. The apparent Km of the carrier was 10 mmol l-1 at pHo 7.35 and 4 mmol l-1 at pHo 6.8. 8. The initial rate of change of pHi in the presence of L-lactate was significantly inhibited 39.1 +/- 6.2% by 2-5 mmol l-1 alpha-cyano-4-hydroxycinnamate (n = 9; P less than 0.05, paired t test). 9. alpha-Cyano-4-hydroxycinnamate had no detectable effect on the initial rate of change of pHi induced by propionate exposure. 10. The initial rate of change of pHi induced by L-lactate was not affected by 20-100 mumol l-1 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS). 11. We conclude that L-lactate crosses the membrane of the frog sartorius muscle with proton equivalents via (1) a carrier-mediated process, and (2) passive diffusion of lactic acid. In the physiological range of L-lactate concentrations and pH the transport process dominates.