Allopurinol is a structural analogue of hypoxanthine and inhibits xanthine oxidase activity. Xanthine oxidase is an enzyme responsible for the production of reactive species during exercise. The aim of the present study was to examine the effect of exhaustive exercise and allopurinol administration on oxidative stress biomarkers in rat liver. Eighty adult male Wistar rats swam until exhaustion in water tanks. The liver was surgically excised pre, immediately after and 5 hours after exercise and the respective time points after allopurinol administration. Xanthine oxidase (XO), total antioxidant capacity (TAC), reduced glutathione (GSH), oxidized glutathione (GSSG), reduced and oxidized glutathione ratio (GSH/GSSG), catalase (CAT), protein carbonyls and thiobarbituric acid-reactive substances (TBARS) were determined in rat liver homoge-nates spectrophotometrically. As expected, allopurinol induced a 4-fold decrease in xanthine oxidase activity. Protein carbonyls concentration, TBARS concentration, catalase activity and GSH/GSSG ratio in rat liver, remained unaffected by exercise. However, exercise alone and allopurinol alone decreased TAC, GSH and GSSG concentrations. In conclusion, both exercise and allopurinol independently decreased TAC concentra-tion in liver which in a long run may compromise the ability of the tissue to counteract the excessive produc-tion of reactive species.