Bicarbonate/chloride (HCO 3؊ /Cl ؊ ) exchangers regulate intracellular pH in the alkaline range. Previously, it has been shown that mouse embryos at the two-cell stage exhibit this activity, but that the otherwise ubiquitous mechanisms for regulating intracellular pH in the acidto-neutral range are undetectable. We have examined mouse embryos during preimplantation development (one-cell zygote through blastocyst) to determine whether HCO 3 ؊ /Cl ؊ exchange activity exists at all stages, whether it is necessary for preimplantation development, and whether messenger RNAs from the known HCO 3 ؊ /Cl ؊ exchanger genes are expressed. We have found that all stages of preimplantation embryo have detectable HCO 3 ؊ /Cl ؊ exchange activity. In addition, inhibition of this activity with the stilbene anion exchange inhibitor DIDS (4,4-diisothiocyanostilbene-2,2-disulfonic acid) disrupts intracellular pH homeostasis and markedly inhibits embryo development from the twocell stage to blastocysts in culture under conditions of moderately high external pH. Finally, mRNA encoding two members of the band 3-related AE anion exchanger gene family are expressed in preimplantation embryos.