As with most cells, bovine pre-elongation embryos are highly dependent on oxidative phosphorylation as the primary energy production pathway (that is, ATP generating pathway; Thompson et al., 1996). This is particularly so during pre-compaction development, during which it is estimated that approximately 90% of all ATP is derived from oxidation (Thompson et al., 1996). During compaction and blastulation, the demand for ATP increases, to allow increases in protein synthesis (Thompson et al., 1998) and the activity of the Na+–K+-ATPase (Leese, 1991), which forms an osmotic potential across the trophectoderm, producing the blastocoel. The increased demand for ATP causes increases in consumption of the major substrates, including oxygen and pyruvate (Thompson et al., 1996), amino acids (Partridge and Leese, 1996) and glucose (Thompson et al., 1996). However, most of the glucose that is metabolized by ruminant embryos at the blastocyst stage is accounted for by glycolysis, and lactate is the end-product and is transported from the embryo to the surrounding medium. Only a small amount of glucose is oxidized, mostly via the pentose-phosphate pathway for ribose formation, rather than by the tricarboxylic acid cycle (Rieger and Guay, 1988; Thompson et al., 1991, 1996; Gardner et al., 1993). However, the increase in glucose consumption is such that the contribution of glycolysis alone to ATP production increases from approximately 4–8% to 15–18% between pre-and post-compaction stages, respectively, in an environment in which O2 is abundant (Thompson et al., 1996). Several studies have indicated that the O2 tension of the reproductive tract decreases as an embryo passes from the oviduct to the uterine cavity (Brown and Mattner, 1984; Fischer and Bavister, 1993). Furthermore, of the little information available concerning the metabolism of inner cell mass (ICM) tissue, it appears that this is more glycolytic in activity than that of the trophectoderm (Hewitson and Leese, 1993). These findings indicate that there is a shift in the metabolic pathway preference for embryonic ATP production from oxidative phosphorylation to glycolysis, to correspond with development within the uterine cavity, an environment