Female mice were dosed with a number of chemical clastogens. The dosed females were then mated 28.5, 6.5, and 0.5 days after dosing, corresponding to various developmental stages of oogenesis (ie, early pre-antral to late antral stages). One-cell embryos derived from these mating were then isolated and analysed for structural and/or numerical chromosome aberrations. Chromosome damage, either structural or numerical, was induced only in oocytes at the late antral stages of oogenesis (i.e. when females were dosed 0.5 days prior to mating). There were few chromosome aberrations in embryos obtained from females dosed 6.5 or 28.5 days prior to mating. The results show that the analysis of one-cell embryos derived from the germ cells of dosed females is a potentially useful test system for the identification of chemically induced structural and numerical chromosome aberrations. However, the technically demanding methods involved in the preparation of these one-cell embryos prevent their use for routine screening purposes.