The production of a luteotrophic hormone by ectopically transplanted rat blastocysts was demonstrated by transfer of blastocysts to the kidneys of cyclic and pseudopregnant mice and isogeneic rats. Pseudopregnancy was induced in nearly all cyclic mice bearing trophoblastic outgrowths but not in cyclic rats. In both mice and rats prolongation of pseudopregnancy was observed following ectopic transplantation of rat blastocysts at the time of normal egg implantation. It is concluded that ectopically developing rat trophoblastic tissue produces a luteotrophic hormone.