This study compares the developmental potential of enucleated mouse parthenogenones that received pronuclei from fertilized eggs with those that received nuclei from late two-cell embryos. The proportion of reconstituted parthenogenones, which received pronuclei at the one-cell stage, that developed to blastocysts in vitro and to live fetuses after transfer to recipients was significantly lower than that of reconstituted control eggs. However, the in vitro and in vivo developmental potential of reconstituted parthenogenones that received nuclei at the two-cell stage from fertilized late two-cell embryos was not different from that of reconstituted control eggs. These results were contrary to those reported previously by Mann and Lovell-Badge (1984), who showed that parthenogenetic eggs receiving pronuclei from fertilized eggs developed well both in vitro and in vivo.