To clarify the causes of low viability of hamster embryos following in vitro culture, the present study compared the distributions of mitochondria and the cytoskeleton in embryos grown in vivo and in vitro. Hamster 2- and 4-cell embryos were characterized by perinuclear clustering of mitochondria, the degree of which was almost the same for in vivo and in vitro embryos. In the cell cortex and cell-to-cell contact region, however, microfilaments were located less densely in in vitro embryos than in in vivo ones. The nucleus moved towards the apex of the blastomere at the late 8-cell stage, when embryos begin the process of compaction. The density of mitochondria seemed to increase in the cell-to-cell contact region during this cellular rearrangement. Mitochondria were concentrated at the perinuclear region of in-vivo 8-cell embryos, whereas they were diffused into the subcortical region of in-vitro 8-cell embryos. Such a diffusion pattern of mitochondrial distribution was also noted in the morulae and blastocysts grown in vitro. These results show that both mitochondrial translocation and cytoskeletal reorganization did not proceed normally in the hamster embryos cultured in vitro, probably resulting in decreased viability of these embryos.