Chromosome separation in meiosis I is different from those in mitosis and meiosis II in that homologs separate from each other in the former while sisters do so in the latter. We show here that meiosis-specific cohesin subunit Rec8 in mouse oocytes shows essentially the same pattern of localization to those reported in yeasts and mammalian spermatocytes; Rec8 along chromosome arm (armRec8) is lost at the metaphase I-to-anaphase I transition, although centromeric Rec8 (cenRec8) is maintained until the onset of anaphase II. Suppression of the loss of armRec8 by microinjection of anti-Rec8 antibody into the oocytes inhibits homolog separation but not the first polar body emission (cytokinesis). Similarly, the injection of anti-Rec8 antibody into metaphase II oocytes prevents sister separation in anaphase II after oocyte activation. These data demonstrate that the loss of armRec8 and cenRec8 is required for separation of homologs and sisters, respectively, but both are not required for other late mitotic events such as spindle elongation and cytokinesis in mouse oocytes. Further, by using some inhibitors for spindle assembly, proteasome and Topoisomerase II and overexpression of Securin, we propose that loss of armRec8 (homolog separation) and cytokinesis are suppressed until anaphase I by Securin whose destruction is regulated by spindle checkpoint-proteasome pathway, and that Topoisomerase II is required for homolog separation independently from such pathway.