Nonenzymatic collagen cross-linking occurs in a variety of connective tissues as a result of formation of advanced glycosylation end products. Diabetes and aging significantly increase levels of nonenzymatic collagen cross-linking in connective tissues. This study was undertaken to determine whether nonenzymatic collagen cross-linking occurs in rat cortical bone and if these levels are increased in diabetic and aged rats. Collagen-linked fluorescence, a measurement of nonenzymatic collagen cross-linking, was significantly increased in rat cortical bone with diabetes and age. In addition, incubation of bone powder with glucose resulted in a similar increase in collagen-linked fluorescence. These changes in bone collagen may contribute to alterations observed in bone with diabetes and age by influencing bone cell function and the ability of the matrix to be responsive to bone cells.