SUMMARY Lactic acid is produced as an end product of glycolysis in rainbow trout white muscle following exhaustive exercise. The metabolically produced lactic acid causes an intramuscular acidosis that must be cleared, either via net transport out of the muscle or by conversion to glycogen,thereby replenishing the muscle energy store. Trout muscle has been shown to retain lactate and utilise it as a substrate for in situ glycogen resynthesis. The giant sarcolemmal vesicle preparation was used to characterise the potential for lactate loss from white muscle of rainbow trout. Minimal lactate loss was expected due to the requirement within the intramuscular compartment of lactate for glycogen resynthesis. The sarcolemma was found to be highly resistant to lactate loss, with efflux rates approximately 500-fold lower than influx rates [0.049±0.006 nmol mg -1 min -1 ( N =21) versus 26.4±6.3 nmol mg -1 min -1 ( N =5),respectively, at 25 mmol l -1 lactate concentration]. Lactate efflux was linear over the range 10-250 mmol l -1 lactate, and greatest under conditions when intravesicular pH was lower than extravesicular pH, but was unaffected by α-cyano-4-hydroxycinnamate, a known inhibitor of lactate transport. These results suggest that lactate is relatively impermeant to the trout white muscle membrane and any lactate loss occurs via passive diffusion. This resistance to lactate diffusion can explain why trout muscle retains lactate post-exercise, despite transmembrane gradients that should favour net efflux.