These experiments assessed the role that exercise-induced alterations in glutathione metabolism might play in the acceleration of breast cancer development in rats subjected to nonexhaustive levels of treadmill running. The influence of intensity and duration of treadmill exercise on total and oxidized levels of liver glutathione was evaluated. Nonexhaustive exercise failed to alter the hepatic concentration of total glutathione. However, increasing exercise intensity resulted in tissue accumulation of oxidized glutathione, and an increase in the ratio of oxidized to total glutathione, indicative of an elevated level of intracellular oxidation. Although alterations in glutathione metabolism do not appear to be directly related to the effect of exercise on tumorigenesis, the role of oxidant-induced intracellular damage in this process is being investigated.