The last 25 years have seen an ever-growing use of the erythrocyte micronucleus test for measuring damage to mammalian chromosomes in vivo. In addition, staining micronuclei with antikinetochore antibodies from CREST serum discriminates aneugenic from clastogenic damage. The use of the micronucleus test in rats, however, has been problematic because the spleen of adult rats efficiently removes micronucleated erythrocytes from the blood. In the present study, we have treated 5-day-old rats with either X-rays (a clastogen) or vinblastine (an aneugen) and measured micronuclei in erythrocytes from the blood and liver. Each treatment increased the frequency of micronuclei in both tissues, with the percentages of CREST-staining micronuclei reflecting the mechanism of micronucleus induction by the two agents. The results indicate that performing the micronucleus assay in the liver and peripheral blood of 5-day-old rats may be a useful approach for detecting the in vivo genotoxicity of chemical and physical agents.