The effect of in vitro exposure to the herbicide chloridazon on the induction of chromosome aberrations (CA) and sister chromatid exchanges (SCE) was studied in cultured sheep peripheral lymphocytes. A positive clastogenic effect was observed in chloridazon-treated cultures at a dose of 7 x 10(-4) M both in the presence and absence of the S9 fraction, but no significant increase of chromosome breaks was seen at lower doses (7 x 10(-6) M and 7 x 10(-5) M, respectively). A clear dose-dependence and significant differences were found in chloridazon potency to induce SCEs. Induction of cell cycle delays as compared to the controls was not observed.