In trained (long-distance runners) and untrained volunteers aged 22 to 57 years, basal plasma glutathione (GSH) concentration decreased significantly with increasing age. In most age groups basal GSH concentration of oxidized glutathione (GSSG) and lipid peroxides (LPO) was not influenced either by age or by training. Following acute physical exercise (bicycle ergometric test), which was maximal for each person and thus different in trained and untrained persons, plasma GSH, GSSG, and LPO concentrations were not changed at all in untrained volunteers. In trained persons, however, GSH concentration was significantly decreased by about 30%. Surprisingly, we observed a decrease in plasma LPO content by about 40%. GSSG content was not altered. The comparison with some physical-physiological parameters indicates that the changes in GSH and LPO concentrations depend on the extent of acute physical exercise rather than on previous training. Alterations of GSH and LPO in plasma could be of physiologic significance.