Rat lung slices produce lactate until the concentration in the medium greatly exceeds the concentration we found in blood from anesthetized rats. In contrast, slices of liver, kidney, or cells washed from the lung will consume lactate until the concentration in the incubation medium is less than the blood concentration. Furthermore, the lactate-pyruvate ratio in lung tissue was higher than in other tissues. Halothane, pentobarbital sodium, or nitrogen with 5% carbon dioxide as the gas phase increased the lactate production from lung tissue. However, palmitate, acetate, and paraquat did not significantly affect the lactate production.