The present study reports a biopsy technique on decompacted late mouse morulae. Zygotes were collected from hyperstimulated F1 hybrids (C57BL6j females x CBAca males) and cultured in modified Earle's balanced salt solution supplemented with 0.5% bovine serum albumin until the late morula stage (92 h post administration of human chorionic gonadotrophin). Decompaction was obtained by exposure of the embryos to Ca(2+)-Mg(2+)-free phosphate-buffered saline (PBS) or an aqueous solution of ethylene-diaminetetraacetic acid (EDTA) and glycine. Using micromanipulation, a single blastomere was aspirated without removal or softening of the zona. The impact of the different decompaction procedures and the biopsy technique was studied by vital staining, by in-vitro culture up to the early egg-cylinder staged and by the recovery of living mice after transfer to pseudopregnant foster mothers. Our investigations revealed no impact of decompaction and biopsy on immediate viability, as assessed by fluorescein diacetate staining. A significant reduction (P less than 0.05) in the number of mouse morulae that reached the early egg-cylinder stage in vitro was observed after the biopsy procedure. We observed that after this microbiopsy technique, successful pregnancies can be obtained but at a lower percentage compared to controls (P less than 0.01)