Controversy exists concerning the effect of aging on replicating cell systems. This review summarizes a number of studies which indicate that both in vivo and in vitro, cell replication is significantly altered during aging. In vitro, studies of both human lymphocytes and fibroblasts indicated that a number of replication kinetic parameters are influenced by the age of the cell donor. In vivo, the application of the bromodeoxyuridine-(BrdU)-differential chromatid staining techniques to the analysis of cellular replication kinetics has permitted us to demonstrate that cellular replication is also significantly diminished with aging in mouse and rat cell populations. Therefore, both in vivo and in vitro in human as well as rodent cell populations, the rates of cellular replication are significantly decreased with cellular aging.