A high proportion (> 70%) of mouse and hamster oocytes exposed for 3-5 min to 1.5 M dimethylsulfoxide (DMSO) and washed briefly in 3.9 M DMSO before vitrification in 6.0 M DMSO appeared morphologically normal on recovery. Significantly fewer (< 46%) mouse oocytes appeared normal when the time of exposure to 1.5 M DMSO was reduced to 1 min or less. The rate of fertilization in vitro of vitrified oocytes was reduced compared to the rate for untreated controls (mouse: 79% vs. 94%; hamster: 73% vs. 87%). After removal of the zona pellucida, fertilization was similar in vitrified and control hamster oocytes inseminated with hamster (> 90%) or human (21% vs. 23%) sperm. Sperm nuclear decondensation and pronuclear formation appeared to be delayed in the cytoplasm of vitrified hamster oocytes. Seventy-nine percent of 2-cell-stage mouse embryos derived from vitrified oocytes implanted after transfer to pseudopregnant recipients, but only 40% developed to normal fetuses compared to 61% of controls. The reason for this high rate of postimplantation loss is unknown.