In recent years, considerable attention has been focused on the rôle of lactate dehydrogenase and its isoenzymes in reproduction and early mammalian development. The study of the developmental progressions of these multiple enzyme forms has greatly advanced our understanding of differential gene control in early ontogeny (Cahn, Kaplan, Levine & Zwilling, 1962; Markert, 1963) and, in addition, it has become evident that the principal reactions of this enzyme (i.e. lactate and pyruvate) are of prime importance as energy sources during the initial stages of cell multiplication following fertilization (Brinster, 1967). Again, the lactate dehydrogenase activities associated with mammalian ova before implantation have been reported to be elevated to a level many times higher than at any other stage of morphogenesis (Brinster, 1965). These findings, with the attendant implications, have established the major