Mouse embryos of 8-cell stage and compacted morulae(approximately 16 cells) containing different number of blastomeres were bisected and cultured in vitro to determine the developmental potentials of the divided embryos compared with those of unmanipulated control embryos. The results were as follows. 1. Micromanipulation was performed successfully by means of a simple manipulator which holds a fine glass, needle, without the use of any micro-instruments for support. 2. The percentage of bisected morulae with 7-9 blastomeres that developed to eu-blastocyst was 94.1% while only 64.8% of the bisected 8-cell embryos with 4 blastomeres developed to eublastocysts (p<0.05). 3. The percentage of eu-blastocysts decreased, while that of pseudoblastocysts and trophectodermal vesicle increased as the number of blastomeres decreased in the bisected embryos of the two stages. 4. The time of the blastocoele re-formation of the bisected and control embryos was not significantly different in morulae stage embryos, but it was significantly delayed in the 8-cell stage embryos (Eu-B, Pseudo-B) compared with control embryos (P<0.01, P<0.05 respectively).