Kinetics of cell replication was compared in regenerating livers of Mus musculus at ages ranging between 6 and 32 months. Incorporation of [3H]-thymidine into DNA and autoradiographic analysis showed that the maximal extent of DNA replication following partial hepatectomy became delayed with age. Furthermore, the total fraction of parenchymal and nonparenchymal cells in S phase at different intervals during regeneration diminished as mice aged. The specific activity of DNA polymerase-alpha, the putative replicative enzyme, declined progressively during aging. The specific activity of DNA polymerase-beta, the purported repair enzyme, declined to an appreciably lesser extent during the lifespan of the mouse. No evidence was found for the appearance of a specific inhibitor of polymerase-alpha in senescent mouse liver. Also, the bulk of the activities of both hepatic DNA polymerase-alpha and -beta remained localized in the cell nucleus throughout the lifetime of the animal and were mainly associated with chromatin.