Many data suggest an involvement of toxic oxygen radicals in the termination of endurance to muscle fatigue. Being reduced glutathione (GSH), an efficient intracellular physiological antioxidant, experiments have been performed to discover whether exogenous GSH modifies endurance to exhaustive swimming in mice. GSH was administered to mice as a single dose (250, 500, 750 or 1000 mg/kg i.p.) or as repeated doses (250 mg/kg i.p. once a day during 7 days) 10 min before a swimming test to exhaustion. GSH 500, 750 and 1000 mg/kg, increased endurance to swimming by respectively 102.4%, 120.0% and 140.7%. GSH 250 mg/kg did not affect endurance when injected in a single dose but increased it by 103.7% when injected once a day for 7 days.