Background: The objective of this study was to investigate whether the steroid hormone(s) secretedfrom cumulus-oocyte complexes (COCs) is a prerequisite for bovine oocyte maturation and cumulusexpansion using aminoglutethimide (AGT), a P450 cholesterol side-chain cleavage inhibitor.Methods: In experiment 1, COCs were cultured in maturation medium with various concentrations ofAGT for 22 h to determine the effective concentration of AGT to inhibit steroid hormone secretion,meiotic maturation and cumulus expansion. In experiment 2, COCs were cultured in conditioned medium(CM) and TCM-199 medium with or without 10 mM AGT to check whether steroid hormones secretedfrom COCs were responsible for oocyte maturation and cumulus expansion. Experiments 3 and 4 werecarried out to determine whether exogenous progesterone or estradiol-17beta was able to overcome theinhibitory effects of AGT on oocytes maturation and cumulus expansion. COCs cultured in 10 mM AGT-containing medium supplemented with various concentrations of progesterone or estradiol-17beta for 22h were examined for oocyte maturation and cumulus expansion. Results: Experiment 1 showed that a concentration of 10 mM AGT in medium was sufficient to blocksteroid hormone secretion, oocyte maturation and cumulus expansion, and that these inhibitory effectswere fully reversible. In experiment 2, the addition of 10 mM AGT to CM did not significantly preventoocyte maturation and cumulus expansion, implying that CM contains the steroid hormone(s) secretedfrom COCs, which are closely associated with oocyte maturation and cumulus expansion. The results inexperiments 3 and 4 demonstrated that the addition of any concentration of progesterone or estradiol-17beta in the medium did not reduce the inhibitory effects of AGT on oocyte maturation and cumulusexpansion.Conclusion: Our results indicate that bovine oocytes surrounded by cumulus cells are prevented frommaturation and cumulus expansion through the inhibition of steroid secretion due to AGT, and that theseinhibitory effects of AGT on oocyte maturation and cumulus expansions can not be overcome by theaddition of either progesterone or estradiol-17beta in the medium. These observations suggest that somesteroid hormone(s) other than P4 and E2 secreted from bovine COCs is essential for their meioticmaturation and cumulus expansion.