To establish a paraquat-resistant Wistar rat strain, we carried out continuous sister-brother mating among rats that survived high-dose intraperitoneal administration of paraquat dichloride (360 mg/kg). The percentages of paraquat-resistant rats among wild rats and among the fifth-generations were 7.1% and 20.6%, respectively. After high-dose paraquat administration, the serum paraquat concentration in sensitive rats was much higher than that in paraquat-resistant rats. The cytosol fraction of liver from paraquat-resistant rats had higher paraquat- and diquat-metabolizing activities than that of liver from paraquat-sensitive rats. By contrast, microsomal fractions from livers of paraquat-resistant and paraquat-sensitive rats had no paraquat- or diquat-metabolizing activity. This paraquat/diquat-metabolizing enzyme was partially purified from paraquat-resistant rat liver cytosol using affinity chromatography for diquat. At the end of the purification procedure, rat liver diquat-metabolizing enzyme was purified 1154-fold to a final specific activity of 32.32 mol/h/mg protein, and an overall recovery of about 0.46% was obtained. This enzyme oxidized diquat to diquat-dipyridone during overnight incubation at 37 degrees C, but only metabolized traces of paraquat. The molecular mass of the enzyme was estimated as 190 kDa, and its isoelectric point of it was 4.6-4.7. Kinetic study revealed the values of K(m) and V(max) to be 35.0 micromol/l and 0.81 micromol/h/ml, respectively.