The relationship between cytoskeleton and morphology of cumulus granulosa cells in expanding bovine oocyte-cumulus complexes (OCCs) cultured in vitro has been investigated by the means of indirect immunofluorescence and transmission electron microscopy. The round-shaped cells in unstimulated control OCCs displayed a homogeneous distribution of cytoskeletal networks and cytoplasmic organelles. Luteinizing hormone (LH) stimulation caused the redistribution of microfilaments (MFs), accelerated the development of Golgi apparatus, and led to the generation of lipid droplets in cumulus cells. These changes culminated in the elongation and polarization of cumulus cells and in the extension of the cytoplasmic networks of microtubules (MTs) and intermediate filaments (IFs) into the newly formed cytoplasmic projections. The culture of OCCs in the presence of microfilament disruptor cytochalasin B prevented cumulus expansion, formation of cellular projections and cell elongation and suppressed the development of the Golgi apparatus. On the contrary, cytochalasin had no effect on the abundance and distribution of lipid droplets and on the integrity of IFs and MTs. The present data support the hypothesis that the response of cumulus granulosa cells to LH is partially mediated by F-actin.