An autoradiographic study of femoral-fracture repair using tritiated thymidine was made on thirty-five female mice, eighteen months of age.

Assessment of the periosteal labeling index was obtaibed from eight hours to fourteen days after fracture.

Results showed that the intial proliferative response to trauma occurs after eight hours and that the response extends throughout the shaft. After forty-eight hours the periosteal response away from the fracture site, having reached a maximum labeling index of 0.110, decreases rapidly so that the non-fractured level (0.005) is reached by the fourth day. At the fracture site the labeling index continues to increase to a maximum labeling index of 0.180 at four days, then decreases rapidly, reaching a plateau of approximately 0.045.

In comparing the present results with those of young mice it is apparent that the difference in fracture healing is quantitative and not qualitative. Some old-looking, flat periosteal osteogenic cells are still capable of desoxyribonucleic acid synthesis and respond to trauma at the same time as similar cells in five-week-old mice. Flat, elongated periosteal cells, in response to trauma, altered their morphological characteristics so that in many instances they were indistinguishable from cells of younger mice.

In older mice the formation of the internal callus progresses more rapidly than the formation of the external callus. This appears to be largely due to the proliferative response of the osteogenic cells within the intertrabecular spaces near the fractured ends.

The over-all labeling of periosteal cells in older mice is less than that encountered in young mice.