Preimplantation embryonic development is associated with a change in preference in energy metabolism pathways. Although oxidative phosphorylation is obligatory in most species throughout preimplantation development, an increasing role for energy derived from glycolysis is associated with compaction and blastulation. Such a shift in metabolic pathway preference is desirable as the embryo faces an increasingly hypoxic environment in utero. We hypothesize that this shift in metabolic preference is associated with a change in the reduction-oxidation (REDOX) state within the embryo, affecting not only the energy production required for development, but also the activity of REDOX-sensitive transcription factors, which may alter gene expression patterns. Shifts in intracellular REDOX state may also contribute to spatial differences in cell activity, especially after compaction, and perhaps even major embryonic events such as fertilization, genome activation and cellular differentiation.