It is found out that the acceleration of preimplantation morphogenesis is observed in mouse chimeras between single two-cell-stage blastomeres of BALB/c and [BALB/c x C57B1/6] x C57B1/6 (F2) embryos. The paired aggregation of blastomeres of two-cell stage made F2-BALB/c chimeras to start cavitation 8 +/- 2 hours earlier than F2<-->F2 and BALB/c-BALB/c control embryos. We have demonstrated for the first time the 'chimeric' heterosis on preimplantation stages. Possible approaches to the future studies of this phenomenon are discussed.