It was shown in this experiment that many systematic functions in male Wistar rats gradually declined during aging. Compared with young rats, thymus weight or the weight index decreased significantly in the 13- and 24-month rats (P less than 0.001). Age-dependent decreases of proliferation rate of splenic T lymphocyte, plasma testosterone (T) and estradiol (E2) level, concentration of cytochrome P-450 and activity of mixed functional oxidase (MFO) in liver microsomes were also observed.