The objective of this contract is to provide a colony of hybrid rats that range in age from 1 to 36 months that will be available to investigators throughout the United States for the conduct of gerontological research. As opposed to most other biomedical disciplines requiring rodent models, gerontologic research involves rodent models representative of complete lifespan. These animals must be characterized genetically and microbiologically an maintained under the strictest of conditions in order to minimize environmental and/or disease variables so that one can differentiate differences due to age, environment or disease at the conclusion of studies. This particular contract will provide a supply of inbred Brown Norway (BN) rats and the Fl hybrid of the inbred Fischer 344 (F344) female mated to the BN.