The effect of potassium bromate (KBr03)on preimplantation mouse embryo development was evaluated in vivo. Pregnant mice were treated with unique dose of KBr03 (68,5 mg/kg of corporal weight; n= 8) and a control (C) provided with distilled water (n= 7) on day 1; at the fourth day of pregnancy, females were sacrificed, embryos were flushed from oviducts and uterine horns for evaluation. KBr03 causes a delay in the embryonic development, 76,9±7,8 y 11,2±5,5 percent of blastocyst and morulaes respectively in the control group comparing with 34,8±11,2 y 49,3±11,9 percent of the same relation in the treated group, showing significative difference (p<0,05). Concerning to the embryonic quality, an increase in the percentage of low quality embryos (degree III and degenerated) in the treated group is observed, but this difference is not statistically significant (p>0,05). In conclusion we can say that KBr03 produces a harmful effect on the embryo causing a delay on its development.