The effects of aging on hepatic and pulmonary cytosolic glutathione S-transferase activities and concentrations were investigated in male Fischer 344 rats 3, 12, and 24 months old. Column isoelectric focusing of liver cytosol indicated that activities, measured with six epoxide substrates, of isozymes E, C, and B increased from 3 to 12 months of age and decreased from 3 to 24 months of age whereas activities of isozyme A were increased in the old group. With lung cytosol, activities of isozymes E, C, and B were decreased whereas isozyme A activities were increased in the old group. Purification to homogeneity of individual isozymes from liver and lung cytosol from each age group indicated that catalytic properties of isozymes were not altered by aging. Immunotitrations of hepatic and pulmonary cytosol from each age group showed that changes in the concentrations of these isozymes occurred with aging: heptic isozymes E, C, and B increased from 3 to 12 months of age and then decreased by 24 months of age, whereas isozymes A and AA were increased by 24 months of age. Pulmonary isozymes E, C, and B followed a pattern of decline from 3 to 24 months of age, whereas isozyme A concentrations were unchanged with increasing age. Analysis of these changes suggested that subunits Ya and Yc as one group and Yb and Y'b as another followed similar increases (from 3 to 12 months) and decreases (from 12 to 24 months) in the liver and that these subunits showed consistent decreases with age in the lung.