These experiments were carried out to develop new techniques necessary for elevating utilization efficiency of high quality embryos and production of cloned animals by nuclear transplantation. Pronuclei removed from the mouse embryos of pronuclear stage by micromanipulation procedures were transplanted to enucleated mouse embryos of same development stage by similiar micromanipulation procedures. Fusion of transplanted pronuclei to the cytoplasm of recipient embryos was mediated by Sendai Vitus. Of 194 pairs of pronuclei transplanted, 126 (64.9%) pairs were fused to the cytoplasm of enucleated recipient embryos. The number of embryos developed to 2-cell embryos, morula and blastocyst after the fusion of transplanted nuclei were 108 (85.7%), 94 (74.6%) and 88 (69.8%), respectively.