The formation of paramagnetic chromium in the liver of male mice dosed with K2Cr2O7 (10, 20, and 40 mg Cr/kg) by a single ip injection was investigated by electron spin resonance (ESR) spectrometry. Both Cr(V) and Cr(III) complexes were detected in the mice livers at 15 min to 12 hr after Cr(VI) injection. The time course (15 min-12 hr) for the formation of paramagnetic Cr revealed that the hepatic levels of Cr(V) complexes decreased quickly during the first hour but decreased more slowly over the next 11 hr. However, in contrast to Cr(V), Cr(III) complexes appeared to persist for 12 hr after Cr(VI) treatment based on figures of Cr(III) signal. Thus, Cr(III) may be the ultimate form following reduction of Cr(VI) in liver. The total Cr content in liver of mice dosed with dichromate was also increased in a dose-dependent fashion (10-40 mg Cr/kg). However, Cr content in liver remained at similar levels for 15 min-6 hr, and slightly decreased at 12 hr after Cr(VI) injection. Under the same experimental conditions, hepatotoxicity, as estimated by the increase of serum ornithine carbamyl transferase activity, appeared at 3 hr after 20 and 40 mg Cr/kg of Cr(VI) injection, while 10 mg Cr/kg of Cr(VI) produced no hepatotoxicity even at 12 hr. Predosing with phenobarbital, which increased the hepatic levels of cytochrome P450, resulted in a decrease of the levels of Cr(V) and in a small increase of Cr content, without affecting Cr(VI) hepatotoxicity. On the other hand, pretreatment with buthionine sulfoximine, which depleted hepatic glutathione (GSH) levels, caused a decrease of Cr(VI) hepatotoxicity, but the levels of Cr(V) and Cr in the liver remained unchanged. These results demonstrated that in vivo formation of paramagnetic Cr, in particular Cr(V), in liver of mice is clearly detected and quantified by ESR spectrometry and that hepatic levels of cytochrome P450 and GSH are associated with the induction of biological effects by Cr(VI) in liver in vivo. The results also suggested that the formation of Cr(V) was not the only mechanism involved in the induction of hepatotoxicity by Cr(VI) compounds.