Rabbit ovarian oocytes co-cultured with granulosa cells were transferred for fertilization to the oviducts of recipient does. Oocytes with cumulus cells and membrana granulosa cells were isolated before or 3 h after hCG injection. Granulosa cells did not prevent the resumption of meiosis but the time sequence of nuclear maturation was retarded by about 3 h. When oocytes were isolated from FSH-stimulated ovaries and cultured with only their cumulus cells or with cumulus and granulosa cells of the same origin, non-decondensed sperm heads were detected in about 30% oocytes after fertilization. Culture of FSH-stimulated oocytes with granulosa cells isolated 3 h after hCG injection substantially enhanced the development of male pronuclei to 88.2% and regular cleavage to 85.2% 10 h and 20 h after transfer, respectively. Further improvement was observed if the oocytes and their co-cultured granulosa cells had been exposed to hCG for 3 h.