Cortical bone collagen was obtained by decalcifying femoral bones from 2-, 5-, 15-, and 25-month-old male rats. Collagen specimens were cut longitudinal to the long axis of the femur and tested mechanically. The maximum load (ultimate strength) and maximum slope of the load-strain curve (maximum stiffness) were found to decrease with age. The age-related reduction in the mechanical parameters resulted from a change in the mechanical strength of the constituent collagen and a change in the morphology of the bone tissue. In vitro aging, produced by incubating for 0-5 months, did not change the mechanical strength of bone collagen specimens obtained from 2-month-old rats. The changes in the mechanical characteristics of bone collagen, accompanying in vivo and in vitro aging are the opposite of those observed in soft tissue collagen. In the latter there is an increase in the mechanical strength during in vivo and in vitro aging.