As a preliminary experiment to establish the process on the sexing of mouse embryos by chromosomal analysis, present studies were carried out with inbred (ICR, C57BL) and F1 hybrid [(ICR${ imes}$C57BL) = F1 ${ imes}$ ICR] mice to investigate the blastomere numbers and mitotic indices (M.I.) to the developmental stage of embryos recovered, the optimum periods of anti-mitotic agent administration, the successful rates of sexing and sex-ratio. The results obtained were summarized as follows: 1. The blastomere numbers (mean${pm}$S.E.) of the morula and blastocyst were 18${pm}$0.4 and 54${pm}$0.7, respectively. 2. Whereas the M.I. of F1 hybrid (16${pm}$0.2%) was higher than that fo inbred ICR (15${pm}$0.1%) and C57BL (12${pm}$0.6%) in the different strains, the morula (7${pm}$0.6%) was higher than that of blastocyst (6${pm}$0.4%) in the case of embryo stages. 3. Following to anti-mitotic agents treated, the M.I. of embryos cultured with Colcemid (17${pm}$1.1%) was superior to that fo embryos cultured with Velban (12${pm}$0.9%) and the Colcemid injection (7${pm}$0.4%). 4. The successful rate of sexing in the blastocyst (38.7%; 124/320) was superior to the morula (35.9%; 52/145), and the F1 hybrid (48.1%) was higher than that of inbred ICR (42.4%) and C57 BL (28.2%). 5. In the successful rate of sexing to the methods of administration, the embryos cultured with Colcemid (46.0%) was superior to that of embryos cultured with Velban (39.0%) and the Colcemid injection (38.8%). 6. Of 98 embryos sexed after culture with Colcemid, 89(90.8%) were observed between 2 and 4 hrs. In the case of Velban treatment, 83.1% (74/89) was observed between 2$frac{1}{2}$ and 4$frac{1}{2}$ hrs. 7. Out of 761 prepared embryos it was possible to sex 311; 157 were male and 154 were female, i.e.a sex-ratio of 50% a, pp.oximately.