Female B6C3F1 mice and Fischer 344 rats were exposed to styrene at nominal concentrations of 125, 250 and 500 ppm by inhalation for 6 h per day for 14 consecutive days. One day after the final exposure, murine peripheral blood lymphocytes, spleen and lungs were removed, and the cells were cultured for analysis of chromosomal aberrations, micronucleus induction (using the cytochalasin B-block method) and sister chromatid exchange. Peripheral blood smears were scored for micronucleus induction in normochromatic erythrocytes. For the rats, peripheral blood lymphocytes were cultured for analyses of sister chromatid exchange, chromosomal aberrations and micronuclei in cytochalasin B-induced binucleated cells and were also examined in the single-cell gel assay for analysis of DNA strand breakage under alkaline conditions. Bone-marrow smears were made from femurs of rats for analysis of micronucleus induction in normochromatic erythrocytes. Small but statistically significant concentration-related increases in the frequency of sister chromatid exchange were seen in both mice and rats in all cell types examined. No statistically significant concentration-related increase in chromosomal aberration or micronucleus induction frequencies were observed in either species, and there was no significant increase in DNA strand breakage in peripheral blood lymphocytes from exposed rats. These results indicate that styrene is a weak inducer of sister chromatid exchange in vivo when administration to rodents by inhalation.