In this report we demonstrate the successful in vitro culture offertilised embryos from 1-cell to blastocyst stage, albeit in astrain-dependent fashion. We report procedures for the enucleationof rat oocytes; nuclear transfer by injection of nuclei (NT) fromadult rat cumulus cells, rat primary embryonic fibroblasts and geneticallymodified rat fibroblasts; and activation resulting in advancedpreimplantation development. Blastocyst stage rat embryos were obtainedafter in vitro culture of nuclear transfer zygotes at similar frequencieswith each of these nuclear donor cell types. Transfer of NT embryos tosurrogate mothers leads to implantation of 24% of the zygotes. Theseresults suggest that the nuclei of cultured rat cells, even followinggenetic modification, can be reprogrammed to support early embryonicdevelopment, which is a prerequisite to cloning the rat.