These experiments were carried out to develop new technique essential for the production of chimeric animals. Embryos obtained from inbred ICR, C3H and C57BL mouse were subjected to in vitro aggregation and aggregated chimeric embryos were developed to blastocysts and then transferred to pseudopregnant recipients. The results obtained were summarized as follows; 1. Production rate of live young following transfer of intact mouse morula to the uterus of recipient mouse on day 2,3 and 4 of pseudopregnancy wre 24.7, 61.9 and 21.8%, respectively, and those of intact morula transferred on 3,4 and 5 days of pseudopregnancy were 58.8, 63.4 and 17.3%, respectively. 2. Chimeric mouse embryos produced by in vitro aggregation of two embryos of the same stage and developed to morula or blastocyst by in vitro culture were transferred to pseudopregnat recipients and 12.2 and 9.2% of chimeric morulas and blastocysts transferred were developed to live chimeric young. 3. Among transferred multiple chimeric mouse embryos produced by in vitro aggregation of 3 to 7 embryos, 9.6% were developed to live chimeric young. 4. Of total 236 chimeric embryos prouced by the aggregation of two embryos of different stage, 78.8% were developed to normal chimeric blastocyst by in vitro culture.