Nuclei of diplotene (dictyate) primordial oocytes (PO) were transferred to metaphase II oocytes and to activated mouse oocytes using cell fusion techniques. In a metaphase II oocyte, the PO nucleus condenses within 2-3 h to bivalents which become arranged on the first meiotic spindle. After oocyte activation, homologous chromosomes segregate between the oocyte and the first polar body, and a diploid pronucleus-like nucleus reforms from the one set of dyads. This nucleus condenses in the first embryonic mitosis into 40 'somatic' chromosomes which coexist in the common metaphase plate with 20 somatic chromosomes originating from the female pronucleus. Shortening of the time between fusion and activation of about 1 h prevents bivalent differentiation. The PO nucleus condenses only partially and reforms, after oocyte activation, a pronucleus-like nucleus. This nucleus gives rise at the first embryonic mitosis to 20 bivalents which coexist with 20 somatic chromosomes originating from the female pronucleus. A PO nucleus introduced into an activated egg completes the first cell cycle as an intact interphase nucleus. It never condenses in the first embryonic mitosis into bivalents, and undergoes only initial condensation (preceding bivalent differentiation). These results indicate that: (1) condensation into bivalents, meiotic spindle formation and first meiotic division can be greatly accelerated by the introduction of an early diplotene (dictyate) oocyte nucleus into a metaphase II oocyte, and (2) depending on whether the diplotene nucleus enters the first embryonic (mitotic) cell cycle after just initiating or after completing the first meiosis, it gives rise at the first cleavage division to meiotic (bivalents) or 'somatic' chromosomes respectively.