Mature mouse oocytes were cryopreserved by vitrification in 6 M dimethyl sulfoxide (VS). After warming they were either artificially activated with strontium (Sr2+), and the incidence of chromosome non-disjunction was assessed at first cleavage metaphase; or they were fertilized in vitro, and postimplantation survival was examined at Day 15 of gestation. Similar proportions of vitrified and freshly collected oocytes were activated with Sr2+ (75% and 82%, respectively). The majority of activated oocytes extruded the second polar body and formed a single pronucleus ( > 90%). When the exposure time to VS was increased from 90 to 110 sec without cooling, a significant proportion of activated oocytes arrested at the pronuclear stage (30%), and chromosome condensation did not occur. The frequency of aneuploidy in vitrified and control oocytes was similar, but when exposure to VS without cooling was extended, aneuploidy and second polar body retention were significantly higher than those of controls (p < 0.05). The rates of fertilization of vitrified (85%) and control oocytes (92%) did not differ. After transfer, similar proportions of vitrified and control embryos implanted (68-80%) and formed normal fetuses (38-49%). We conclude that vitrification in 6 M dimethyl sulfoxide is a simple and safe procedure for the preservation of mouse oocytes provided that the time of exposure to the cryoprotectant is carefully controlled.