Rodents are commonly used as models for human aging because of their relatively short life span, the ease of obtaining age-specific tissue samples, and lower cost. However, age-associated disease may confound inbred animal studies. For example, numerous physiologically significant lesions, such as chronic nephropathy, are more common in aged F344 rats than in other strains (5). Conversely, F344 x BN F-1 hybrid (F344BN) rats, developed by the NIA for aging research, live considerably longer and have fewer pathologies at any given age versus inbred strains (20). To our knowledge, there are no data regarding the effect of age on bone geometry and mechanics in this strain of rat. Furthermore, caloric restriction (CR) extends the mean and maximal lifespan of animals, significantly reduces age-associated disease, but may have adverse consequences for bone growth and mechanics. Thus we investigated the effects of age and CR on bone geometry and mechanics in the axial and appendicular skeleton of F344BN rats. Ad libitum fed rats were assessed at 8 mo (young adult; n=6), 28 mo (late middle age; n=5), and 36 mo (senescence; n=6). CR rats were assessed at 28 mo (n=6). Tibiae and the sixth lumbar vertebrae (L₆) were dissected and scanned (µCT) to determine geometry, and were tested mechanically. From 8 to 36 mo, there were no significant changes in L₆ geometry, and only the cross sectional moment of inertia changed (increased) with the tibia. CR induced body mass reductions accounted for changes in L₆ load at proportional limit, maximal load, and stiffness (structural properties), but altered tibial structural properties were independent of body mass. In tibiae, geometrical changes dominated alterations in structural properties. Those data demonstrated that whereas aging in ad libitum fed animals induced minor changes in bone mechanics, axial and appendicular bones were adversely influenced by CR in late middle aged animals in different manners.