Aged Lou female rats (33 months) submitted to a self-selection regimen showed a decrease in protein intake (down to 11% of the total intake), whereas mature rats (18 months) selected a high percentage of protein (20% of the total intake) similar to the protein content of the standard diet. To find out if this decrease in protein intake would prevent an observed age-related decrease in proteasome activity, four peptidase activities and oxidized protein degradation were tested with proteasome purified from the liver of 18- and 33-month-old rats. The peptidylglutamyl-peptide hydrolase activity, which is decreased with age for rats fed the standard diet, was restored in the self-selecting old rats to the level observed for the mature rats. Degradation of oxidized glutamine synthetase, which is also decreased with age for rats fed the standard diet, was partly restored. Proteasome from self-selecting old rats showed a slight increase in trypsin-like and chymotrypsin-like activities as compared to proteasome from old rats fed the standard diet. Two-dimensional gel electrophoresis followed by quantitative analysis of the pattern of proteasome subunits revealed an increase in the intensity of two protein spots for proteasome from old rats fed the standard diet as compared with proteasome from either mature rats or self-selecting old rats. These findings may have important implications in aging for proteasome-mediated proteolysis and subsequent accumulation of oxidatively damaged protein.