We have shown previously that porcine cumulus and mural granulosa cells produce a factor that is very similar, if not identical, to the oocyte-derived cumulus expansion-enabling factor (CEEF). Because growth differentiation factor 9 (GDF9) is the most likely candidate for the CEEF, in the present study we tested the hypothesis that GDF9 is expressed not only in oocytes in the pig but also in somatic follicular cells. In addition, we asked whether the relative abundance (RA) of GDF9 mRNA changes in oocytes and/or follicular cells during the periovulatory period or culture of oocyte-cumulus complexes (OCCs) in vitro. Denuded oocytes, OCCs, cumulus, and mural granulosa cells were isolated from growing and preovulatory follicles. Total RNA was extracted from the cells, and reverse transcription-polymerase chain reaction (RT-PCR) was carried out using specific oligonucleotide primers. The RT-PCR resulted in amplification of a product of expected size (277 base pairs) in samples prepared from all follicular cell types. The identity of the RT-PCR products with GDF9 was confirmed by analysis of their nucleotide sequence, which was 88% and 91% identical to human and ovine GDF9, respectively. The RA of GDF9 mRNA in the somatic follicular cells was approximately fourfold lower than in oocytes. Assessment of the RA of GDF9 mRNA during the periovulatory period and during culture and expansion of OCCs in vitro revealed that it remained stable in oocytes and mural granulosa cells and decreased significantly in expanding cumulus cells. We conclude that GDF9 mRNA can be produced by somatic follicular cells in the pig and that cumulus expansion is not preceded or accompanied by an increase in the RA of GDF9 mRNA in any of the tested cell types.