The protein degradation rate of total liver homogenate as well as of the liver cytosol was measured according to the method introduced by GARLICK and coworkers (Biochem. J. 156 (1976) 657-663). The half-lives were 2.85 days (cytosol) and 2.45 days (homogenate) in the group consisting of 29 months old rats and 2.35 days and 2.15 days in the control group consisting of 12 months old rats. The amount of cathepsin D determined in some organs by two independent methods (immunological and enzymic) revealed differences in the group of the old animals. We suggest the accumulation of cross reacting material of cathepsin D in rats of the old group. It is perhaps one of the reasons why a slower protein degradation is observed in old animals.