Lactate accumulation in skeletal muscle is reduced after a period of endurance training. Explanations for this phenomena include the increased oxidative capacity of the muscle, a reduction in lactate production, and increased lactate clearance. Muscle membrane transport of lactate can be seen to be a fundamental aspect of such clearance, and transmembrane lactate flux may well be an important aspect of the training response in skeletal muscle. Therefore, the lactate transport capacity in skeletal muscle sarcolemmal membranes in endurance-trained and sedentary rats was investigated. Training consisted of 6 weeks of progressively increased treadmill exercise. Twenty-four hours before being killed, both the trained and sedentary animals completed a brief exercise bout. Studies of lactate transport (zero-trans) were conducted using highly purified sarcolemmal vesicles. When low concentrations of L-lactate (1 mM) were used a 59.4% increase in lactate transport was observed (P < 0.05). However, when a high concentration of lactate (50 mM) was used no change in lactate transport was found (P > 0.05). Several interpretations are possible for these observations: (1) that there is an alteration in the Km but not the Vmax of the lactate transport system in skeletal muscle membranes; and (2) that specific changes occur in selected isoforms of the lactate transport protein which may co-exist in muscle.