It has been suggested that nucleus replacement (transfer) may be used as an efficient oocyte therapy in order to prevent trans^ mission of mutated mitochondrial DNA from mother to offspring in humans. The essential and not yet answered question is how mitochondria surrounding the karyoplast will be distributed in the newly reconstructed oocytes- In our model expert menls, we have evaluated the distribution of mitochondria in reconstructed immature mouse oocytes when germinal vesicle karyoplasts, with labeled mitochondria, were fused to unlabeled cytoplasts. The penetration of mitochondria from karyoplasts into cytoplasts can be detected almost immediately after the beginning of fusion. In immature reconstructed oocytes, mito¬chondria are first located in the oocyte center but they are homogenously distributed within the whole cytoplasm before the completion of maturation. Fusion of oocytes at different stages of maturation suggests that the speed of mitochondria distri¬bution is cell cycle dependent.