An isolated-perfused tail-trunk preparation was employed to study the influence of transmembrane pH gradient and membrane potential on the transport and distribution of L(+)-lactate (Lac), metabolic H+ (delta Hm+), and related parameters in rainbow trout white muscle after exhaustive exercise. One resting [arterial pH (pHa) approximately 7.9] and four postexercise treatments (pHa approximately 7.4, 7.9, 8.4, and, high K+, pHa approximately 7.9, partially depolarized by 15 mM K+) were examined. Variations in HCO3- concentration (2-18 mM) at a constant PCO2 approximately 2 Torr were used to alter pHa. The elevated intracellular Lac (approximately 50 mM) remained unchanged after 60 min of perfusion because of very low rates of lactate efflux and oxidation. H+, HCO3-, and Lac- distributions were all well out of electrochemical equilibrium. Total CO2 efflux was reduced at high extracellular pH (pHe); alterations in the net driving force on HCO3- may have overshadowed the influence of PCO2 gradients in driving total CO2 efflux. Lac efflux and delta Hm+ flux were completely uncoupled. delta Hm+ flux reacted to both acid-base and electrochemical gradients as delta Hm+ efflux dropped and even reversed when pHe decreased, whereas partial depolarization in conjunction with depressed intracellular pH resulted in elevated delta Hm+ efflux. Lac efflux did not respond to changes in pHe. Changes in Lac efflux corresponded more closely to changes in the Lac- concentration gradient than in the lactic acid gradient. This study provides circumstantial evidence for the involvement of electroneutral mechanisms (i.e., Lac(-)-H+ cotransport and/or Lac-/anion exchange) in lactate efflux, but does not eliminate the possibility of an active transport mechanism contributing to the retention of Lac.