The present experiment was to investigate the effect of gonadotropins (LH and hCG), steroids (estradiol and progesterone) and culture media (TCM 199, Ham-F-12, BMOC-3 and modified KRB) on in vitro maturation of cumulus-enclosed bovine oocytes. Oocytes isolated from follicles of </=5 mm in diameter were cultured for 27 to 30 hours. Dosages of gonadotropins were 10 mug/ml and 2 i.u./ml for LH and hCG, respectively, and that of steroids was 1 mug/ml for both estradiol and progesterone. The study showed that the proportion of matured (metaphase II and a polar body formation) oocytes was significantly affected by the presence or absence of steroids (P < 0.01), but not by the gonadotropins and the media. Estradiol, but not progesterone, significantly increased the rate of oocyte maturation when compared with that of no steroid treatment (47.8%, 61.7% and 55.2% for none, estradiol and progesterone, respectively). These results indicate that steroids, especially estradiol, were the most important factor for bovine oocyte maturation rather than gonadotropins and culture media, and that the effect of the hormones differed in the media tested.