An attempt has been made to improve the early post-implantation development potential of diploid parthenogenetic mouse embryos by transferring parthenogenetic blastocysts to one uterine horn of a pseudopreg nant recipient and a similar number of fertilized embryos to the contralateral horn. In control studies, diploid parthenogenetic embryos were transferred to both uterine horns of appropriate recipients. Unfortunately no obvious advantage appeared to be gained by carrying out the former manoeuvre. A significant improvement in the development potential of the parthenogenones could have indicated that their poor post-implantation survival might have been associated with a deficiency, possibly of hormonal origin, in the functioning of their decidual reaction. However, sufficient somite-containing parthenogenetic embryos were obtained in this study to allow a comparison to be made between them and fertilized embryos that were morphologically at a comparable stage of development. The parthenogenones were found to have a markedly smaller crown-rump length than their fertilized counterparts. A high proportion of both the parthenogenetic and fertilized embryos were subsequently fixed and appropriately stained in order to localize alkaline phosphatase activity. The analysis of this material clearly demonstrated that parthenogenetic mouse embryos are in fact capable of producing primordial germ cells.