To investigate the effect of different concentrations of melatonin on bovine oocytesin vitro maturation, varying concentrations of melatonin (0, 0.01, 1, 100 µM), were included in the the maturation medium. Slaughterhouse derived oocytes were subjected to standard in vitro maturation procedures in high oxygen tension. After culture for 24 h, over 80% of COCs had full cumulus cells expansion. As melatonin concentration increased, the degree of cumulus cells expansion did not changed significantly. Oocytes incubated in 0.01 and 1 µM melatonin-containing media for 24 h, result in 73.11 and 70.68% maturation rate, respectively, which were no different from the control (72.24%). The maturation rate decreased (P < 0.05) significantly when melatonin concentrations was increased from 0.01 to 1 and 100 µM (73.11 vs 70.68% and 65.24% respectively). After culture in melatonin-containing media for 24 h, 100% oocytes achieved GVBD stage in all groups. There were no significant differences among groups at GVBD stage. But oocytes that remained at metaphase-I stage were significant (P < 0.05) different with 100 µM in compare to other groups (21.32 Vs 17.67, 15.68, 16.53). In conclusion in this experiment,melatonin cannot improve cumulus cell expansion and nuclear maturation of bovine oocytes. When concentrations is high, melatonin may affect bovine oocytes meiotic maturation at metaphase-1 stage, but it is improbable melatonin be toxic for bovine oocytes.

Key words: IVM, bovine oocytes, melatonin, meiotic maturation.