Female and male mouse somatic cells were injected into mouse F(1) oocytes. The cells used included cumulus cells (female) and muscle derived fibroblasts (male). The ability of the cells to fertilize oocytes and support embryonic development was examined. Following activation of the injected oocytes, two second polar bodies were extruded and two pronuclei were formed, one derived from the oocyte chromosomes and the other from the somatic cell chromosomes in a similar way to that observed following fertilization with secondary spermatocytes. Both second polar bodies contained DNA. The fertilization rates by cumulus cells were 10-29%. This was dependent on the artificial activation protocol and on the age of the oocytes. Older oocytes recovered 16-17 h after human chorionic gonadotrophin (HCG) injection were more likely to produce two second polar bodies and two pronuclei than young oocytes which were retrieved at 13-14 h after HCG injection (P < 0.01). The fertilization rates with fibroblasts were 29% using the most effective activation regime and aged oocytes. Most (80-90%) of the 'zygotes' produced by somatic cells cleaved to two cells in culture and ~50% reached the morula stage. However, the developmental competence of the embryos to reach blastocysts was limited. The present study demonstrates that mouse somatic cells undergo haploidization when injected into metaphase II oocytes, fertilize oocytes as diploid male germ cells and support preimplantation development to a degree.