There has been much speculation on whether mammalian eggs with two male pronuclei can develop normally. Eggs with two female pronuclei can sometimes develop as far as the 25-somite stage but with only very meagre extraembryonic tissues. We suggested that the genome undergoes specific imprinting during gametogenesis and that some paternal genes may be necessary for normal development of the extraembryonic tissues, in which only the maternal X chromosome remains active. However, the need for the maternal genome for development to term is not yet unequivocally established. The detailed study described here demonstrates that while between 40 and 50% of heterozygous reconstituted eggs with a male and a female pronucleus develop to term, none of the eggs with two male pronuclei does so. Furthermore, embryos in the latter case are very retarded, even though the trophoblast develops relatively well compared with embryos having two female pronuclei. Our combined results indicate that while the paternal genome is essential for the normal development of extraembryonic tissues, the maternal genome may be essential for some stages of embryogenesis.