The effects of endurance training (running 1 h/day at 40 m/min, 10% grade) on net lactate removal at various lactate concentrations were assessed in resting rats by use of constant exogenous lactate infusion (0, 69.3, 123.6, and 175.0 mumol.kg-1.min-1). No consistent difference in resting lactate concentrations, 1.17 +/- 0.09 mM, was observed between control and trained animals with no exogenous infusion of lactate. With increasing lactate infusion rates, control animals demonstrated a twofold greater increase in blood lactate concentration (range 1.2-11.4 mM) compared with trained animals (range 1.0-5.5 mM). This response resulted from a more rapid rise in net lactate removal with changes in blood lactate concentration for trained animals. The estimated maximal reaction velocity for net lactate removal in trained animals was 19% lower than in control animals; however, the Michaelis-Menten constant was greater than 66% lower in trained animals (4 mM) compared with controls (12 mM). Control animals also demonstrated a twofold greater increase in lactate concentration as a function of the tracer-estimated lactate turnover. The ratio of 14CO2 yield to lactate specific activity as a function of total tracer removal was not significantly different between groups, suggesting that the relative contributions of oxidation and gluconeogenesis to lactate removal were similar for both groups. At blood concentrations greater than 1 mM, trained animals achieve higher rates of lactate removal for any given lactate concentration.