The cleavage, metabolism and viability of mouse zygotes were assessed after culture in media of different ionic and metabolite composition. Medium with a high potassium concentration, characteristic of mammalian oviduct fluid, inhibited cleavage and blastocyst formation (P < 0.01). This inhibition was partially alleviated by the removal of phosphate, and subsequently abolished by supplementation with amino acids, vitamins, insulin, epidermal growth factor and transferrin (AVIET). Glucose uptake by cultured blastocysts, measured fluorimetrically, was not affected by the ionic or metabolite composition of the medium, but was significantly reduced by the inclusion of AVIET (P < 0.01). Lactate production was also significantly reduced in the presence of AVIET (P < 0.01). Calculations of metabolic activity revealed that embryos cultured in the presence of AVIET had a glycolytic activity similar to embryos developed in vivo. In contrast, embryos cultured in conventional embryo culture media exhibited an elevated glycolytic activity. Culturing embryos for 4 days in a reduced lactate concentration (4.79 mM), significantly increased fetal development after transfer, compared with embryos cultured in the concentration of lactate present in conventional embryo culture media (23.3 mM; P < 0.01). In contrast, when embryos were transferred on day 3 of culture, significantly more fetuses were obtained from embryos cultured in high levels of lactate (P < 0.01). Supplementation of medium with AVIET significantly increased resultant fetal weights after transfer (P < 0.05). This study demonstrates that different media are required to maintain embryo viability on successive days of culture, and highlights the potential limitations of employing simple salt solutions for the culture of preimplantation mammalian embryos.