The present study investigated the effect of treatment with nocodazole on the efficiency of cell cycle synchronization and development of mouse 4-cefl embryos. In addition, developmental ability of reconstituted embryos that received nuclei from 4-cell embryos synchronized with nocodazole at M phase was examined in vitro and in vivo. (1) When 4-cell blastorneres exposed to culture medium containing l$mu$g /ml nocodazole for 2, 4, 6 and 8 hrs, 40% (40/10l), 75% (l19/159), 85% (87/102) and 97% (155/160) of nuclei were synchronized at M phase, respectively. (2) Treated with nocodazole for 4 hrs, the proportion of 4-cell embryos developed to blastocysts (98%, 60/61) was not significanUy different from that of the control embryos (98%, 196/201). However, the developmental ability of 4-cell embryos treated for 8 (87%, P<0.05)and 12 hrs (76%, P<o.ool) was significantly reduced. (3) The reconstituted embryos that received a metaphase nuclei from 4-cell embryos, which were synchronized at metaphase with nocodazole for 1, 2 and 4 hrs were developed to blastocysts (21 to 31%), respectively. After transfer to recipients females, total of 21 live young were obtained, which derived from the reconstituted embryos. These results should live young be produced from reconstituted embryos which received metaphase nuclei synchronized with nocodazole.