The effect of insulin on in vitro development of early stage tetraploid mouse embryos was examined. Tetraploid embryos were produced at the 2-cell stage by treatment with cytochalasin B for 12 hr, then they were cultured with M16 medium containing 1 μM insulin. Although insulin had no effect on the embryos until 72 hr after hCG injection, at 96 hr the number of cells of insulin (+) tetraploid blastocysts increased 20.2% more than insulin (-) tetraploid embryos. The increase in the number of cells mainly depended on the inner cell mass (ICM), in which insulin (+) cells increase 76.3% more than insulin (-) cells. These effects of insulin on tetraploid embryos were comparable with those on diploid embryos. The present results suggested that the effect of insulin on the cell proliferation of early stage embryos was not affected by tetraploidy, but the increased number of cells in tetraploid blastocysts was significantly smaller than that in diploid embryos.