Oxygen consumption of preimplantation and early postimplantation mouse embryos has been measured using a novel noninvasive ultramicrofluorescence technique, based on an oil-soluble, nontoxic quaternary benzoid compound pyrene, whose fluorescence is quenched in the presence of oxygen. Pyruvate and glucose consumption, lactate production, and glycogen formation from glucose were also measured. Preimplantation mouse embryos of the strain CBA/Ca x C57BL/6 were cultured in groups of 10-30 in 2 microliters of modified M2 medium containing 1 mmol l-1 glucose, 0 mmol l-1 lactate, and 0.33 mmol l-1 pyruvate, for between 4-6 hr. Day 6.5 and 7.5 embryos were cultured singly in 40 microliters M2 medium for between 2-3 hr. Oxygen consumption was detected at all stages of development, including, for the first time, in the early postimplantation embryo. Consumption remained relatively constant from zygote to morula stages before increasing in the blastocyst and day 6.5-7.5 stages. When expressed as QO2 (microliters/mg dry weight/hr), oxygen consumption was relatively constant from the one-cell to morula stages before increasing sharply at the blastocyst stage and declining to preblastocyst levels on days 6.5 and 7.5. Pyruvate was consumed during preimplantation stages, with glucose uptake undetectable until the blastocyst stage. Glucose was the main substrate consumed by the 6.5 and 7.5 day embryo. The proportions of glucose accounted for by lactate appearance were 81%, 86%, and 119% at blastocyst, day 6.5, and day 7.5 stages, respectively. The equivalent figures for glucose incorporated into glycogen were 10.36%, 0.21%, and 0.19%, respectively. The data are consistent with a switch from a metabolism dependent on aerobic respiration during early preimplantation stages to one dependent on both oxidative phosphorylation and aerobic glycolysis at the blastocyst stage, a pattern which is maintained on days 6.5 and 7.5. Our technique for measuring oxygen consumption may have diagnostic potential for selecting viable embryos for transfer following assisted conception techniques in man and domestic animals.