The regulatory enzyme 6-phosphofructo-l-kinase (PFK) controls the key, rate-limiting step in glycolysis. There are 3 known mammalian isoforms termed PFK-muscle (PFK-A), PFK-liver (PFK-B), and PFK-brain (PFK-C) that randomly aggregate to form active homo- and heterotetrameric isozymes with their respective frequencies and kinetic properties contingent upon the presence and concentration of individual subunits. This study utilized RT-PCR and densitometry analyses to characterize the expression patterns of the mRNA for each isoform during mouse preimplantation development. PFK-B is increasingly expressed across these stages with a significant increase in PFK-B transcript between 8-cell (0.425 ± 0.158) and morula (0.579 ± 0.197) stages (p < 0.0005). Neither PFK-A nor PFK-C mRNA was detected at any of the preimplantation stages tested. The statistically significant increase in PFK-B corresponded with the known juncture of the switch from the oxidation of maternally supplied pyruvate to a predominant glycolyticmetabolism. Such timing suggested the direct involvement of elevated PFK-B transcription with an increase in glycolysis. / Department of Biology