The present study was undertaken to determine the efficiency of HVJ treatment and electrofusion for pronuclear transplantation in the mouse. The output voltage and duration of the pulses were fixed to 200 microsec at 10 V or to 150 microsec at 15 V for electrofusion, because the maximum rates of blastomere fusion of 2-cell embryos and development of fused embryos in vitro were obtained under these conditions. Although the proportion of eggs with fused karyoplast (78%) and the fused eggs developed to morulae or blastocysts the proportion of pregnant recipients and young obtained after treatment of fused eggs was not significantly different between these two procedures. It is advised that electrofusion can be used as a fusogenic procedure for pronuclear transplantation in the mouse in some cases where HVJ cannot be applied.