Mouse round spermatids were electrofused with homologous mature oocytes to examine the behaviour of their nuclei within the ooplasm and the abilities of development. A single spermatid was injected in the perivitelline space of a mature oocyte and an electron fusion pulse was given. The best round spermatid-oocyte pairs (RS-O) fusion took place at 20-30 s AC (1 MHz, 50V/cm) followed by a single fusion DC pulse (3 700-3 800 V/cm, 25 mus) and another 30 s AC current. The total survival rate and fusion rate of RS-O were 89.0% (575/646) and 61.9 % (356/575), respectively. 49.2 % (175/356) of fused oocytes developed to 2PN stage. The concentration of Ca(2+) in the fusion medium produced no significant effect on the above targets. The 2PN development rate of the fused RS-O from the oocytes collected 14-16 h after hCG injection was higher than others. 32.6% (57/175) of the 2PN oocytes had fully developed spermatid (male) and oocyte (female) pronuclei. The rest spermatid-derived pronuclei remained small in size throughout the pronuclear stage. 192 fertilized eggs were transferred surgically into the oviducts of the pseudopregnant female mouse. 12 offspring were produced.