When metaphase Il-arrested mouse oocytes (M II) are activated very soon after ovulation, they respond abortively by second polar body extrusion followed by another metaphase arrest (metaphase III, M III; ). The M Il/M III transition resembles the natural transition between the first and second meiotic metaphases (M 1/M II). We observed that a similar sequence of events takes place during these two transitions: after anaphase, a polar body is extruded, the microtubules of the midbody disappear rapidly and a new metaphase spindle forms. The MPM-2 monoclonal antibody (which reacts with phosphorylated proteins associated with the centrosome during M-phase) stains discrete foci of peri-centriolar material only in metaphase arrested oocytes; during both transitional periods, a diffuse staining is observed, suggesting that these centrosoma! proteins are dephosphorylated, as in a normal interphase. However, the chromosomes always remain condensed and an interphase network of microtubules is never observed during the transitional periods. Incorporation of ³²P into proteins increases specifically during the transitional periods. Pulse-chase experiments, after labeling of the oocytes in M phase with ³²P, showed that a 62 kDa phosphoprotein band disappears at the time of polar body extrusion. Histone Hl kinase activity (which reflects the activity of the maturation promoting factor) drops during both transitional periods to the level characteristic of interphase and then increases when the new spindle forms. Both the M 1/M II and M Il/M III transitions require protein synthesis as demonstrated by the effect of puromycin. These results suggest that the two M-phase/M-phase transitions are probably driven by the same molecular mechanism.