To develop, maintain and distribute a standing colony of aged, genetically defined laboratory mice for use by investigators in studies of aging. This colony is to be developed and maintained within controlled and defined barrier environments where animals are monitored and characterized for disease status and markers of genetic purity. Current and projected experiments using rodents to model the aging processes and age-related diseases in humans require animals of defined genotype and controlled environmental and health status. Only with the meticulous, long-term control of genetic and environmental variables is it possible to maintain relevant animal models that may be used to study many of the biological and behavioral processes in aging. Therefore, a continuous supply of genetically defined, well characterized inbred and hybrid laboratory animals is essential for a program of research on aging.