Adult female rats subjected to exercise training in the form of swimming daily (for 20 min initially with gradual increase up to 90 min within a week) for 130 days showed a significant elevation of liver glutathione S-transferase (GST) activity when compared to controls. Electrophoresis and Western blot analysis of cytosolic as well as affinity-purified hepatic glutathione S-transferases revealed the induction of Ya-sized subunit in exercise-trained animals. Reverse-phase HPLC analysis of affinity-purified GSTs further revealed that Ya-sized subunits in control animals consisted of predominantly Ya2, whereas the trained animals displayed a dramatic 4.3-fold increase in Ya1. The GSTs of exercise-trained animals showed increased peroxidase activity when compared to corresponding controls, which is consistent with the changes in sub-unit composition. This could be a response to the physiological oxidative stress induced by physical exercise.