Pituitary LH and FSH are known to be the major regulators of ovarian function. In the last few years, however, there has been evidence that growth hormone (GH) is also involved in ovarian regulation. Therefore, the aim of our study was to elucidate the mechanisms of GH action during in vitro maturation (IVM) of bovine cumulus-oocyte complexes (COCs). As shown by detection of the nuclear cell proliferation-associated antigen Ki-67, COCs matured in vitro in the presence of GH revealed a significantly (P < 0.05) higher proportion of proliferating cumulus cells (12.6%) compared with the COCs matured in the control medium TCM 199 (9.9%). In contrast, the percentage of proliferating cells was not increased by supplementation of the medium with a combination of GH and insulin-like-growth factor I (IGF-I). Apoptosis as determined by TUNEL (terminal doxynucleotidyl transferase mediated dUTP nick-end labeling) was significantly (P < 0.05) reduced in the cumulus cells by GH treatment. COCs matured with a combination of GH and IGF-I revealed the lowest percentage of apoptotic cells (11%). The localization and quantification of the gap junction protein connexin 43 (Cx 43) demonstrated that GH induced a significant decrease in the synthesis of the Cx 43 protein in the cumulus cells. Our results imply that GH increases cumulus expansion by promotion of cell proliferation and inhibition of apoptosis. Whereas the increase in cell proliferation is a direct effect of GH, the antiapoptotic effects of GH during in vitro maturation are modulated by IGF-I. Stimulatory effects of GH on oocyte maturation are correlated with changes in the synthesis of gap junction proteins.