Mouse embryos were biopsied by squeezing, and viability was investigated. The biopsy method includes the following procedures: holding the embryo and partially dissecting the zona pellucida, squeezing a single blastomere from the embryo, sucking the blastomere into a blunt micropipette, and expelling the blastomere from the blunt micropipette. Mean cell numbers for 4-cell biopsied blastocysts (24.7 ± 5.2) were lower than in controls (38.5 ± 6.1) (P<0.0001). Preimplan-tation rates of development to blastocyst for embryos biopsied at the 4-cell and morula stages were 93% and 95%, respectively. In vivo survival rates to full term were similar for embryos biopsied at the 4-cell and morula stages (38% and 45%) and control embryos (46% and 47%, respectively). These results indicate that the squeeze method does not affect in vitro or in vivo viability of biopsied mouse embryos. This procedure may be useful for biopsy of preimplantation embryos for the purpose of sexing or diagnosing defective genes in humans and domestic animals.