This experiment was studied on the times of fetus-death during the gestation period of superovulated mows, and inspectation of implantation and living fetus number and the implantation points were done after 10, 12, 14, 16, 18 days from gestation. The results are as follows: 1. The coputaion rate was 68.1% (47/69) and the gestation rate was 68.1% (32/47) of superovulated mouse. 2. The average number of implantation was resulted like these, the 10 days after was 20.8, 12days after 18. 2, 14 clays after 17.1, 16 days after 17.0 and 18 days after 19.0, while the average number of living fetus were 10 days after 20.8 (100%: living fetus number/implantation number), 12 days after 16.3 (89.9%), 14 days after16.9 (98.3%), 16 days after 10.7 (62.7%) and 18 days after 10.2 (53.7%), and living fetus number was rapidly decreased from 16 days over. 3. The litter size of superovulated mouse was average 6.9 heads and did not over the control plot of which number is aveage 9.7 heads.