The effects of thymic factor D (TFD) on aging were investigated in male Wistar rats at the age of 24 months. Thymic factor, polypeptides extracted from swine thymus, was given to aged rats in the dose of 2 mg.kg-1 sc every other day for 3 months. It was shown that TFU decreased the content of malondialdehyde (MDA) and lipofuscin in the liver of aged rats, accompanied by an increase of glutathione (GSH). Besides, the declined fluidity of hepatic mitochondrial and microsomal membranes in aged rats was recovered to the level of young adult rats by administration of TFD. It is suggested that TFD may possess beneficial effects on retarding the senile metabolic changes in hepatic cells.