POLYSPERMY—the entry of more than one sperm into the vitellus at fertilization—is commonly regarded as a pathological process in mammals, with the implication that it leads to early degeneration of the embryo, and this opinion has been expressed by such authorities as Lams1, Wilson2, and Hartman3. In fact, very little is known concerning polyspermy in mammals, and data on its incidence or consequences are lacking. A study has therefore been made of polyspermy in the rat and rabbit, and a brief account of the results is presented in this communication. A full report is being prepared for publication elsewhere.