In the mouse, removal of the oocyte from oocyte-cumulus complexes (OCC) prevents FSH and EGF induced expansion of the cumulus. This indicates that mouse oocytes produce a soluble factor enabling expansion of the cumulus (CEEF). Porcine oocytes also synthesise and secrete CEEF since they enable the FSH stimulated expansion of mouse oocytectomized complexes (OOX). However, oocytectomy does not affect FSH induced expansion of the porcine and bovine cumuli in vitro. These data indicate that also other cellular compartments of antral follicles in domestic animals are able to produce CEEF. To assess production of CEEF during growth and differentiation of pig oocytes and to find out whether porcine cumulus and mural granulosa cells can secrete CEEF, the culture media were conditioned with denuded oocytes at different stages of their development, by oocytectomized complexes (OOX) representing cumulus cells or by pieces of mural granulosa cells. Media conditioned by growing and fully grown porcine oocytes at GV stage allow the full expansion of the mouse OOX. Granulosa cells isolated from preantral and early antral follicles and cumulus granulosa cells isolated from all stages of follicular development constitutively secreted CEEF under in vitro conditions. Mural granulosa cells of small, medium or preovulatory follicles secreted CEEF after FSH or LH stimulation, respectively. CEEF activity is present in follicular fluid of small follicles, disappears in follicular fluid of medium and large follicles and reappears in preovulatory follicles following hCG injection. The reappearance of CEEF activity in large volumes follicles indicates an involvement of somatic follicular cells in CEEF production. Both autocrine secretion of CEEF by the cumulus cells and paracrine secretion of CEEF by the mural granulosa cells and the oocyte are involved in cumulus expansion in pig follicles. This distinguishes them from murine follicles where the oocyte itself is the sole source of CEEF.