A comparison of results obtained from studies of the intracellular fractions of the tissues of liver, brain and heart of "young" (1-2 months), "old" (24-27 months) and "senile" (34-37 months) rats showed that the ratios of three enzymes, acid phosphatase, beta-N-acetylglucosaminidase and acid RNase of the liver and heart were very similar and their activities decreased with age. On the other hand, the protein content is the supernatant of the liver, and acid DNase activities in the supernatant of the brain increased significantly with age. When the 24-27 month and 34-37 month old rats were compared, the ratios of the total activities of liver beta-N-glucosaminidase and brain acid DNase in the supernatant and the specific activities of brain beta-N-glucosaminidase in the microsomal fraction increased significantly.