A new metabolic reaction of the aldolase condensation between formic acid and acetaldehyde proceeding with the formation of milk acid is detected in the liver of rats. Milk acid has been determined by chemical, enzymic and autoradiographic methods. Homogeneous preparations of the enzyme which catalyzes the mentioned reactions and is called lactate synthase are obtained in the crystalline form. The method for obtaining the lactate synthase from the rat liver is described as well as certain properties of the lactate synthase.