ABSTRACT
The life span and pathologic lesions of an individual rat or population of rats are determined by their genetic background and external environmental factors. Two obvious genetic variables are the strain or stock of rats and their sex. The many combinations of interacting factors determine the longevity and pathologic findings in rats maintained under specific conditions. If one or more factors is changed, the longevity and pathologic lesions may also be changed. Longevity data are needed to compare the life spans of different rat strains and sexes. Most published data indicate that the mean survival for most rat strains is about 28 to 30 months. In addition to longevity data, the baseline neoplastic and nonneoplastic lesions must be determined for each strain of rats. Rats are living organisms similar to man; they develop many age-associated lesions. These age-associated lesions can influence the physiology, biochemistry, and morphology of aging rats.
