Three strains of mice (ICR, B6C3F1 and C 57 BL/6J) of 28, 56 and 112 days old were in superovulation with PMSG and HCG. The techniques of in vitro fertilization, culture, and transplantion of two-cell embryos to pseudopregnant recipients were used. The highest numbers of oocytes were obtained from 28-days old superovulated mice in the three strains, being prominent discrepancy with 56-days and 112-days old mice ( P <0.01). There was no difference of the insemination rate among three strains( P >0.05).Approximately 91% of ova developed to the two-cell stage after in vitro fertilization, and about 51% of these living two-cell embryos from the three different strains developed into foetus through transplantation in recipients, with no discrepancy in rate of pregnancy( P >0.05).\;