1. R. pipiens embryos, and gastrula-arrested hybrid embryos obtained by fertilizing R. pipiens eggs with R. sylvatica sperm, begin to produce lactic acid, without initial lag, as soon as they are deprived of oxygen.2. Provided that they are younger than about 72 hours (18° C.), embryos of both types are able to sustain the initial rate of glycolysis for at least four hours. Older embryos of both types exhibit a linear production of lactic acid for at least one hour.3. The development of glycolytic capacity in R. pipiens embryos occurs in two phases: one of constant glycolytic rate (0-59 hrs.), the other of exponentially increasing glycolytic rate (59-140 hrs.).4. The glycolytic rates of hybrid embryos decline from the normal control value at fertilization to about one-fifth of the control value at 59 hours, then increase to about one-eleventh of the control value at 140 hours.