The ability of sodium meta-arsenite (NaAsO2) to produce genetic damage in vivo has been evaluated in mice by the micronucleus test on bone marrow cells and the dominant lethality and sperm abnormality assays on male germ cells. The dose-related linear increase of micronuclei observed in somatic cells together with the negative results obtained in reproductive cells suggest that NaAsO2, under the conditions of the present experiments, displays clastogenic properties in vivo in laboratory animals, but is unable to produce heritable damage.