The effect of age and food restriction on the hepatic alkaline protease activity of 100,000 x g supernatant has been investigated using 7-, 16-, and 26-month-old Fischer 344 rats. The proteasome, a major component of alkaline protease activity, is activated by sodium dodecyl sulfate (SDS) and this property was exploited to gain insight into the effects of age and food restriction on proteasome activity. Three alkaline protease activities, chymotrypsin-like (ChT-L), trypsin-like (T-L), and peptidylglutamyl peptide hydrolyzing (PGPH) activities were measured. These activities are also commonly used as measurement of proteasomal activities. Basal ChT-L and PGPH activities were not markedly altered by either age or food restriction. The level of T-L activity did not change with age, but was decreased by food restriction. SDS-activated ChT-L activity increased 15% between 7 and 26 months of age and this increase was blocked by food restriction. SDS-activated PGPH activity decreased 40% and the decrease was ameliorated by food restriction. In conclusion, we have shown that the alteration of alkaline protease activities by age and food restriction is not uniform and that the changes observed are likely due to alterations of proteasomal activity. The lack of uniformity in these alterations indicates that any assessment of alkaline protease activity requires the measurement of more than one of the enzymatic activities. Lastly, the first evidence suggesting that age and food restriction can modulate proteasomal activity is presented.