1. Freshly prepared homogenates of R. pipiens embryos contain considerable amounts of lactic acid, aerobically produced from an unknown source. The amount present is an increasing function of the age of the embryos from which the homogenates are made.2. In the presence of suitable exogenous substrates, anaerobic homogenates of R. pipiens embryos, and of hybrid R. pipiens ♀ x R. sylvatica ♂ embryos, produce lactic acid at constant rates for at least two hours. Anaerobic lactic acid production from endogenous substrates occurs at very low rates.3. The relation between homogenate concentration and the rate of lactic acid production under anaerobiosis is linear, but graphs of the relation do not pass through the origin.4. Roughly speaking, the rates at which homogenized embryos glycolyze exogenous glycogen, glucose-1-phosphate, glucose-6-phosphate and fructose-1,6-diphosphate are at least as great for hybrid embryos as for normal R. pipiens controls.5. Exogenous glucose is not glycolyzed by homogenized R. pip...