This study was carried out to investigate the cloning of mouse and rat embryos. Three-hundred females of DDY and ICR strain mice and fifty females of Wistar strain rats were superovulated by intramuscle injection of PMS and HCG. Two-, four-, and eight-cell embryos were dichotomized in vitro by fine glass, needle with hand following the papaya protease digestion of zona pellucida. The results obtained were summarized as follows; 1. The mice were superovulated by injections of 5 IU of PMS and HCG. One, two, and four to eight-cell embryos were mostly obtained from the oviduct at 18 to 24, 48 and 72 hours after injection of HCG. In rat, at 48 hours after HCG, two-cell embryo was 66%, but at 72 hours, four-cell embryo, 44% and eight-cell embryo, 40%, respectively. 2. The percentage of mouse and rat showed superovulation after PMS and HCG treatment was 44 and 30%, respectively. The average number of embryos ovulated per head was 13.7 ± 4.6 and 25.9 ± 8.3 in mouse and rat. There was highly significant differences in the ovulation response and the number of ova ovulated per head between breeds. 3. The rate of dichotomized two-cell embryo in control, zona softened and removed was 0, 48 and 95% in mouse and 0, 44 and 58% in rat, respectively. Rate of the embryos dichotomized without damaged blastomere was 40 and 93% in zona softened and removed, respectively. 4. The rate of dichotomized four-cell mouse embryo was 0, 64 and 96% in control, zona softened and removed and the rate of embryo dichotomized with out damaged blastomeres was 36 and 46%, respectively. The rate of dichotomized four-cell rat embryo in control, zona softened and removed was also 0, 56 and 63%, respectively. The dichotomized rate of embryo was lower in rat than that in mouse. 5. The dichotomized rate of 8-cell mouse embryos was 60 and 82% in zona softened and removed and showed lower dichotomized rate than that of two- and four-cell embryos. In eight-cell mouse embryo, the rate of embryo dichotomized with out damaged blastomees was 17 and 34% in zona softened and removed.