Changes in plasma glutathione (reduced form: GSH; oxidised form: GSSG) and lipid peroxides (LPO) levels occur with aging, training and acute physical exercise. Sources of plasma GSH, GSSG and LPO include the liver and skeletal muscle. Aging appears to be accelerated because of a decrease in the antioxidant capacity of tissues reflected in a decreased plasma GSH level. This age-dependent change could be partly compensated by physical training. Skeletal muscle appears to be able to deliver GSH into circulation with the adaptation of muscle to exercise training reflected in an increased plasma GSH level in the trained subject. Decreased plasma GSH concentration following physical exercise demonstrates increased GSH consumption in skeletal muscle resulting in a reduced export rate from muscle into plasma. The GSH system is able to effectively protect tissues against lipid peroxidation initiated by oxygen-derived free radicals produced in the intermediate metabolism during exercise. It can be assumed that the rate of this free radical production is a function of oxygen flow through organ and muscle tissue.