The dynamics of changes in the spectrum of lactate dehydrogenase was studied in the early pre- and postimplantation development in the CBA mice by the method of ultramicrophoresis. In the preimplantation embryos only LDH-1 was found during the first 5 days of development. The LDH-5 first appears by the 5th day of development and is preserved at all subsequent stages. The LDH spectrum of 5--7 days old embryos is represented by LDH-1 and LDH-5 and a very small amount of hybrid forms. The distribution of isozymes in the spectrum does not correspond to the binomial one. The LDH-1 is completely absent from 8--11 days old embryos and reappears in some tissues beginning from the 13th day of development. The distribution of isozymes in the LDH spectra from the 8th day of development is close to the binomial one.