Porcine embryonic fibroblasts in M-phase were isolated from confluent cultures by selective detachment and fused to homologous metaphase II enucleated oocytes. The fusion products were analysed by light and electron microscopy (EM). The fusion treatment, per se, did not induce the disappearance of the maturation promoting factor (MPF) in the cloned embryos and the transferred chromatin remained condensed for up to 10 h. Spontaneous decondensation started after this time and, at about 20 h post-induction of fusion, a single large nucleus was present in the cytoplasm. The prolonged exposure of introduced chromatin to MPF resulted in a total morphological remodelling as assessed by the nucleolar morphology observed by EM in all the specimens.