The nuclei from one-, two-, four-, and eight-cell rat embryos were transferred into enucleated zygotes and development to term of the reconstituted embryos, after transfer to recipients, was examined. The rate of electrofusion of enucleated zygotes with karyoplasts was quite high (81-92%), but the fusion rate was only 8% when Sendai virus (HVJ) was used. A large percentage of the enucleated zygotes that received pronuclear karyoplasts developed to the twocell stage in vitro (89%) and to term (22%) after transfer to recipient rats. Although 67%, 5752, and 67% of enucleated zygotes receiving nuclei from two-, four-and eight-cell embryos, respectively, developed to the two-cell stage, they failed to develop to term after transfer.
Since a successful procedure for nuclear transplantation of mouse embryos was reported by McGrath and Solter ('83), the technique has been employed not only for studies of production of cloned animals (McGrath and Solter, '84a; Tsunoda et al., '87a) but also for studies of the development of gynogenetic and androgenetic eggs (Surani et al., '84, '86) and analysis of maternal inherited effects (McGrath and Solter, '84b; Mann, '86; Nakamura and Tsunoda, '87). These studies involved only cells from mice as the donors and recipients of nuclear material. None of the enucleated mouse zygotes into which nuclei from four-cell or more advanced embryos were placed were able to develop to blastocysts in vitro (McGrath and Salter, '84a; Tsunoda et al., ,874. Sheep (Willadsen, '86) and bovine (Prather et al., '87) offspring have been produced from blastomeres of eight-cell and nineto 15-cell embryos, respectively, after nuclear transplantation. However, data on the developmental ability of reconstituted embryos are lacking for other species. The present study, therefore, was undertaken to examine nuclear transplantation of rat embryos by the method of McGrath and Solter ('83) and to assess the developmental ability of enucleated zygotes reconstituted with karyoplasts from zygotes and from two-cell to eight-cell embryos.