The germ cell and somatic cell compartments of newborn rat and mouse ovaries, which contain only primordial stage follicles, were completely exchanged and reaggregated to produce xenogeneic chimeric ovaries. The reaggregated ovaries were grafted beneath the renal capsules of ovariectomized SCID mice to develop for periods up to 21 days. Xenogeneic follicles developed with essentially normal morphological characteristics. Both rat and mouse oocytes with species-specific characteristics grew within follicles that were composed of somatic cells exclusively of the alternative species. Rat oocytes grown in mouse follicles became competent to resume meiosis, and progressed to metaphase II when they were removed from follicles and cultured. In addition, mouse oocytes grown in rat follicles underwent fertilization and preimplantation development in vitro, and developed to term after embryos were transferred to pseudopregnant mouse foster mothers. Therefore, despite an estimated 11 million years of divergent evolution, oocytes and somatic cells of rat and mouse ovaries can be exchanged and can produce functional oocytes. It is concluded that factors involved in oocyte-somatic cell interactions necessary to support oocyte development and appropriate differentiation of the oocyte-associated granulosa cells are conserved between rats and mice. Moreover, although granulosa cells play important roles in oocyte development, the development of species-specific characteristics of oocytes occurs without apparent modification by a xenogeneic follicular environment.