It is well known that developmental delay or arrest occurs before implantation in mamals, which have undergone in vitro culture. Recently, these phenomena are being attributed to oxygen free radicals, and successful cell culture are being obtained by lowering the oxygen environment of in vitro culture. This is due to the fact that the oxygen concentration in the fallopian tube is around 5%, which is lower than the room air 20% concentraition for in vitro culture. Superoxide dismutase(SOD), which are free radical scavenger, were added to early embryo mice culture and compared the free radical level at each stage of development with that of culture in which free radical scavenging agent was not added. The highest free radical level in control group observed in the late 2-cell stage and was markedly decreased if SOD added to culture media. Blastulation rate, hatching rate, and blastomere count as the parameters of embryo development was significantly increased in the experimental group. This study suggests the possibility of improvement in in-vitro embryo culture with media including free radical scavengers.