PURPOSE: The present study focused on the metabolic properties involved in lactate shuttle in slow-type and fast-type skeletal muscles from aged rats, including monocarboxylate transporter (MCT) 1 and MCT4, to explore the mechanisms for greater fatigue resistance and less lactate accumulation during prolonged muscle contraction in old rats. METHODS: Soleus (SOL) and extensor digitorum longus (EDL) muscles were obtained from old rats, young and hindlimb-suspended rats, and young control rats. Protein expression of MCT1 and MCT4 and enzymatic activities of glycolysis and mitochondrial oxidation were measured. RESULTS: MCT1 expression was lower in SOL from of old rats than that from young control and young suspension groups. MCT4 expression was also lower in EDL from old rats than in that of young control and young suspension group. The ratio of lactate dehydrogenase to citrate synthase activities was lower in SOL from young control than that from old and young suspension groups. In EDL, the ratio was lower in old group than in young control and young suspension groups. CONCLUSION: Aging can change metabolic properties in skeletal muscle to facilitate lactate oxidation in fast-type muscle and can increase fatigue resistance. These changes are as a consequence of aging rather than of inactivity.