The purpose of the proposed contract is to develop, maintain and distribute animals from a standing colony of aged rats of various genotypes for use by investigators in studies of aging. Numerous investigators, supported by the NIA, other institutes, and the Veteran's Administration, use NIA rats in their research and are dependent upon NIA for access to a supply of aged rats for their research. Provision for a supply of aged rats is essential to the achievement of NIA research objectives by NIA grantees.