In most studies of aging of rodents, investigators have utilized male animals. Recently, it has been recognized that there are sex differences in age-associated changes of some physiological parameters. However, the breadth and, more importantly, the mechanisms of i these sex-based differences have not been explored. The purpose of this proposed Animal Core Resource is to provide female mice and rats of various ages to investigators currently involved in aging research, to determine whether or not there is a sex-associated difference in the age-associated parameter that they are investigating. The Animal Core Resource will not only provide female animals to expand their research, but will also characterize all female animals, either purchased by the Core or by the investigator, as to the status of the estrus cycle and sex hormone profile. This assessment will provide definitive information on the role of sex hormones in sex-based differences observed during aging. Once areas have been identified that clearly demonstrate sex differences in age-associated changes, the Core will provide hormonally deprived animals to further explore the role of endocrine hormones in this sex-based effect. In addition to providing female mice to investigators currently funded for aging research, both female and male rodents will be made available to-Junior Investigators (who may also be funded through the Research Development Core) as well as to senior investigators who are interested in expanding their funded research to include gerontology but do not have the funds to obtain preliminary data in aged animals. Finally, the Animal Core Resource will facilitate both maximal utilization of fresh tissues and maximal deposition of unused tissues in close cooperation with the Cell and Tissue Core by coordinating experiments among the various investigators who will be using the Animal Core Resource. The activities of the Animal Core Resource will not only allow expansion of ongoing aging research, but also will provide the impetus for new research in the basic scientific aspects of aging.