Evidence is now emerging that the oocyte plays a role in the development and function of granulosa cells. This study focuses on the role of the oocyte in the proliferation of (1) undifferentiated granulosa cells from preantral follicles and (2) more differentiated mural granulosa cells and cumulus granulosa cells from antral follicles. Preantral follicles were isolated from 12-day-old mice, and mural granulosa cells and oocyte-cumulus complexes were obtained from gonadotropin-primed 22-day-old mice. Cell proliferation was quantified by autoradiographic determination of the ³H-thymidine labeling index. To determine the role of the oocyte in granulosa cell proliferation, oocyte-cumulus cell complexes and preantral follicles were oocytectomized (OOX), oocytectomy being a microsurgical procedure that removes the oocyte while retaining the three-dimensional structure of the complex or follicle. Mural granulosa cells as well as intact and OOX complexes and follicles were cultured with or without FSH in unconditioned medium or oocyte-conditioned medium (1 oocyte/μl of medium). Preantral follicles were cultured for 4 days, after which ³H-thymidine was added to each group for a further 24 h. Mural granulosa cells were cultured as monolayers for an equilibration period of 24 h and then treated for a 48-h period, with ³H-thymidine added for the last 24 h. Oocyte-cumulus cell complexes were incubated for 4 h and then ³H-thymidine was added to each group for an additional 3-h period. FSH and/or oocyte-conditioned medium caused an increase in the labeling index of mural granulosa cells in monolayer culture; however, no differences were found among treatment groups. Both in oocyte-cumulus cell complexes from antral follicles and in preantral follicles, the removal of the oocyte resulted in a decrease in the labeling index compared with intact samples (23% reduction in OOX complexes and 71% reduction in OOX preantral follicles). Oocyte-conditioned medium reversed this effect to bring the labeling index of OOX complexes and follicles to the level of intact samples. Oocyte-conditioned medium had no effect on intact oocyte-cumulus cell complexes but resulted in a doubling of the labeling index in intact preantral follicles. FSH had a negative effect on proliferation in oocyte-cumulus cell complexes, causing a reduction of 32% in intact complexes and 27% in OOX cumulus complexes. The treatment of OOX complexes with FSH prevented the stimulatory effect of conditioned medium. No effect of FSH was observed on preantral follicles in these short-term cultures. These results indicate that the oocytes secreted one or more factors that promoted granulosa cell proliferation by both the relatively undifferentiated granulosa cells of preantral ovarian follicles and the more differentiated cumulus and mural granulosa cells of antral follicles. The terminally differentiated cumulus cells treated with FSH, however, failed to respond to the oocyte-derived proliferation factor(s); this observation indicates that the factor can act on granulosa cells only at specific stages of their development.