Synchronous or asynchronous chimaeras were produced by transplanting a single nucleus of two- and eight-cell embryos from CD-1xCD-1 or BALB/CxBALB/C albino strains into one enucleated blastomere of a late F1 (C57/BLxCBA) x F1 two-cell embryo. The cytoplasmic volume of the blastomere was reduced in some instances by 50%. These chimaeric embryos were cultured in vitro and transferred to pseudopregnant recipients. The distribution of each component to the pups and to the day-10 embryos after transfer to recipients was determined by examining their coat color and by glucose phosphate isomerase analysis, respectively. The contribution of progeny of the nuclear-transplanted cell with nonreduced cytoplast to the pups was 83% when synchronous; this proportion decreased to 43% when asynchronous because the progeny tended to migrate to the trophoblast and/or to the primitive endoderm. When the recipient cytoplast was reduced by 50%, the contribution of the nuclear-transplanted cell progeny to the pups was 79% when synchronous and 80% when asynchronous. This shows that allogenic exchange of a single nucleus at the two-cell stage by nuclear transfer is an effective procedure for producing highly asynchronous mouse chimaeras and suggests that larger and advanced blastomeres tend to be excluded from the inner cell mass of the embryo, but smaller, advanced blastomeres do not.