The capacity of sister blastomeres of mouse embryos for induced fusion changed during the 2-cell stage. It was at low level (24%) at the early 2-cell stage, increased and reached 98.5% at the middle 2-cell stage and fell sharply to 31% at the late 2-cell stage. At the time corresponding to the G2/Mphase of the cell cycle the blastomeres fused in only 8% of cases. Vital staining of 2-cell embryos by rhodamine 123 showed that the mitochondria were dispersed throughout the cytoplasm with a ringlike (around the nucleus) or spot-like (over the metaphase plate) concentration in the centre of each blastomere. At the periphery of blastomeres the mitochondrial content was low. The behaviour of the mitochondria reflected the subsequent events of structural and functional integration of the sister blastomeres under induced fusion: a discernible boundary between partners during 30 min after electrofusion or 1 h after fusion with polyethylene glycol; movement of the two ‘rings’ to the centre of the blastomere fusion products (BFP) to form one large bright ‘spot’ over the common metaphase plate; mitochondria outlining the shape of the spindle and connection between sister blastomeres until completion of the first mitosis of BFP The data obtained suggest that fusion of the blastomeres does not lead to extensive changes in the hybrid cytoplasm and integration of nuclear material is taking place only at metaphase stage Cytogenetic examination of BFP at the 2-cell stage confirmed reconstruction of the tetraploid embryos and found that sister blastomeres of such embryos could asynchronously enter the next cleavage division similarly to normal diploid 2-cell embryos.