Rats and mice are used in gerontological research primarily because of their relatively short life spans, ease of handling, and the relatively low costs of production and maintenance under controlled environmental conditions of large number of rodents as compared to larger laboratory animal species. They are being used as models for studying intrinsic aging processes, processes that give rise to diseases associated with aging, and the influence of environmental factors on these processes. Contrary to the situation in man, longitudinal studies in rodents can be conducted under well controlled environmental conditions. It has been shown that multiple pathology, the hallmark of aging in man, also occurs in inbred strains of rodents. Some of these lesions are genetically determined and some of them are randomly distributed amongst members of the same inbred strain. Serial killing experiments are necessary to obtain information on the time of development of these lesions in order to interpret properly the outcome of investigations. Furthermore, it has been shown that a considerable variation can exist in the observed maximum ages of the longest-lived animals in cohorts of rats kept under well controlled conditions. For this reason, caution should be exercised in interpreting data from studies which claim maximum lifespan prolongation.