To assist in defining the mechanism(s) by which the activity of hepatic glutathione S-transferase (GST) is decreased in obese rodents, the cytosolic concentrations of individual GST isozymes were determined by high-performance liquid chromatography. For this purpose, liver cytosols from 8- and 16-week-old obese yellow Avy/a and lean black a/a male and female mice of the inbred VY/WffC3Hf/Nctr-Avy strain were assayed. Obese yellow males contained less hepatic GT-9.0 than lean black males; however, there were no differences between the obese and lean females. GT-9.0 concentration, which is induced by testosterone, was several-fold higher in males than in females, regardless of genotype or body weight. No differences in concentrations of other isozymes were detected. Hepatic GST activity towards 1-chloro-2,4-dinitrobenzene was significantly higher in lean males than in obese males; however, there were no differences between obese and lean females. Lean males had higher activity than lean females; but obese males and females had similar enzyme activities. These changes in enzyme activity can be accounted for by the changes in GT-9.0 concentration measured by HPLC. Lung and testes cytosols were also assayed for GST isozyme concentrations. No differences in any isozyme concentration were found between the sexes or the genotypes in the lung or between genotypes in the testes.