Pyruvate is added to all media used for human in vitro fertilization and embryo culture, but its function(s) in the early embryo is unknown. We tested the possibility that pyruvate can act as an oxidizable energy source by measuring the consumption of pyruvate and oxygen by Day 2 and Day 3 human embryos, using microfluorometric techniques. Oxygen consumption (19.6 pmol/embryo per hour) could account for the oxidation of only 56% of the pyruvate consumed (13.9 pmol/embryo per hour). Oxygen was also consumed in the absence of exogenous substrates. Lactate appeared in the incubation medium with pyruvate (0.47 mM) as sole exogenous substrate at a rate of 12.1 pmol/embryo per hour, at a similar rate (10.85 pmol/embryo per hour) in the presence of 1 mM glucose and 0.47 mM pyruvate, and at 2.25 pmol/embryo per hour in the absence of exogenous substrates, suggesting that a high proportion of the pyruvate taken up by early human embryos is converted to lactate. Pyruvate uptake in the presence of UK5099, a pyruvate transport inhibitor, was reduced to 10% of control values, consistent with the presence of the monocarboxylate carrier in the human embryo plasma membrane.