It is well known that developmental delay or arrest occurs before implantation in mammals, which have undergone in vitro culture. Recently, these phenomenon 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% concentration for in vitro culture. Rutin, which is a free radical scavenger, was added to early embryo mice culture and compared the free radical level at blastocyst stage with that of different culture conditions, and found that free radical level was markedly decreased. Also, there was increased embryo development with decreasing free radical levels in the experimental group, and there was significant increase in the blastulation rate and blastomere count. This study therefore suggests the possibility of improved in in-vitro embryo culture.