This study was undertaken to examine the developmental capacity of reconstituted mouse embryos, and the influences of nucleus and cytoplasm on the development of these embryos following reciprocal pronuclear transplantation between in vitro 2-cell blocked and nonblocked embryos. Karyoplast containing pronuclei was transferred into the perivitelline space of the enucleated zygote and fused to cytoplasm with electrofusion. Maximum fusion rate was obtained when a field strength of 1.5 kV/cm was used. The fusion rates were high (86.2 +/- 3.2 to 90.6 +/- 2.0%) regardless of the strains of donor nucleus and recipient cytoplasm. Developmental rates of reconstituted embryos to the blastocyst stage, which were similar to that of the F1 (C57BL/6J x CBA) control were high when F1 embryos were used as the cytoplasm recipients (88.8 +/- 1.5 and 91.9 +/- 2.0%). When ICR embryos were used as the recipient cytoplasm, developmental rates were significantly reduced (71.5 +/- 2.9 and 54.1 +/- 3.2%), and affected by the source of nucleus. There were no significant differences in the cell number of embryos that developed to blastocysts and in the developmental rates to live young among the embryos reconstituted with different nuclei and cytoplasm, and the ICR control. The results of this study show that the development of reconstituted embryos is hardly affected by nuclear transplantation and electrofusion procedures. It is indicated that the recipient cytoplasm, rather than the donor nucleus, has the greater influence on the in vitro development of the reconstituted embryos to the blastocyst stage.