The cellular DNA damage response (DDR) entails the activation of ATM, ATR and/or DNA PK protein kinases that causes modifications of proteins including Chk1, Chk2 and 53BP1, aggregation of DDR proteins into foci, and activation of p53. The DDR is thought to be required for initiation and maintenance of cellular senescence. Potentially senescent cells with DNA damage foci occur in large numbers in vivo with many diseases, but, with the exception of mammalian dermis, there is little evidence for that with normal aging. After experimental induction of cellular senescence in the livers of juvenile mice, there was robust expression of DDR markers in hepatocytes at 1 week; however, by 7 weeks, activation of ATM/ATR kinase targets was limited, although cells with DNA damage foci were present. An analysis of hepatocytes of aged, 22-month-old mice, not experimentally exposed to genotoxins, showed limited activation of ATM/ATR targets, though high numbers of cells with DNA damage foci were found, similar to that seen many weeks after artificial senescence induction in young mice. Based on senescence heterochromatin and SA ss Gal assays of the 22-month-old mouse liver, more than 20% of hepatocytes were potentially senescent, though only some components of the DDR were enriched.