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Low power He-Ne laser irradiation was applied to immature bovine oocytes to investigate the incidence of unreduced oocytes (diploid oocytes) during the first meiotic division in vitro. Immature bovine oocytes from cows killed at a slaughterhouse were irradiated with He-Ne laser irradiation at 0.05 and 0.25 J/cm(2) exposures. An oocyte group was left untreated serving as the control group. All oocytes were matured in TCM-199 medium supplemented with 10% fetal calf serum (FCS). Unreduced oocyte percentages obtained in the irradiated oocytes group were significantly higher (p<0.01 and p<0.001, respectively) than those of the control group. Furthermore, the laser-treated oocytes showed a degeneration rate significantly higher (p<0.01 and p<0.001, respectively) than those observed in the control group. It is concluded that the application of He-Ne laser irradiation at 0.05 and 0.25 J/cm(2) exposures increased the incidence of unreduced oocytes and the percentage of degenerated oocytes during the in vitro meiosis process of immature bovine oocytes.