We investigated the effect and the fate of an extremely high amount of orally administered lactic acid in rats. The dosed amount of lactic acid, 390 mg per 200 g body weight (30 times higher than that normally detected in the stomach of rats), was determined from the results of observation of acute toxicity of lactic acid in rats. Six hours after the administration of excess lactic acid together with 10 muCi of L-[U-14C]Lactic acid and 10 muCi of D-[U-14C]lactic acid, rats were sacrificed and the pH of the blood, the amount of lactic acid in each organ, L-lactate dehydrogenase (LDH) and some other enzyme activities and incorporation of radioactivities in each fraction of certain organs were measured. The control rats were given the labeled lactic acid and the same volume of water in place of cold lactic acid. Significant decrease of blood pH (delta- pH = 0.14) and increase of blood lactic acid concentration (2-fold) were observed. However, these differences were no longer observed at 24hr after the administration. The amount of lactic acid degraded to expired CO2 was 42.4% in the experimental group, whereas it was 61.3% in the control group. Radioactivities incorporated into protein and lipid fractions in the experimental group were higher than those in the control group, 3.8 and 4.9 times, respectively. It was suggested that an extremely high amount of orally administered lactic acid was utilized as an energy source, and that an excess of lactic acid was incorporated into protein and lipid in addition to degradation into CO2.