Spermatozoa of 10, 15 and 20-wk-old KM mice were frozen and stored at--196 degrees C. After thawing, intact superovulation oocytes were inseminated in vitro with relative high motility frozen/thawed mouse spermatozoa. Some 2-cell embryos obtained by in vitro fertilization were transferred to pseudopregnant recipients and 47%-56% of them developed into live-born offspring. Some 2-cell embryos were incubated in vitro and 57% of them progressed to the blastocyst stage. Others were cryopreserved by simple vitrification procedure, and most of them (about 80%) were morphologically normal after warming, and 50% of the cryopreserved embryos transferred to recipients had developed into live pups. These results indicated that there was no significant difference when compared with embryos obtained from oocytes inseminated with fresh sperm.