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Non-invasive microanalytical methods have been devised to study the energy metabolism of single human preimplantation embryos. Pyruvate, which is added routinely to all media used to culture human embryos, is consumed throughout the preimplantation period, with glucose assuming an increasing role at embryo compaction and blastocyst formation. All of the glucose consumed may be accounted for by the appearance of lactate in the incubation medium. The enzyme hexokinase may be involved in regulating this aerobic glycolysis. There is considerable indirect evidence for the utilisation of endogenous as opposed to exogenous energy substrates, the most likely candidate being protein. Information on early human embryo metabolism is likely to find application in a number of areas: these include the improvement of techniques for assisted human conception, notably in the selection of embryos for transfer following In Vitro Fertilisation; the diagnosis of genetic defects at the preimplantation stage; increased understanding of the causes of implantation failure and miscarriage, and the development of novel post-coital contraceptives.