Estimation, by ten-minute clearance of 85Sr, of the blood flow in dogs' tibiae with healing fractures showed an increase in flow the first day after fracture and, at approximately two weeks, a maximum of almost six times mean normal flow. This maximum was followed by a decrease to approximately three and one-half times that of normal at three weeks, maintenance of this level until eight to ten weeks, and then return to more normal values as healing and remodeling neared completion at twelve weeks. In a fracture with delayed union, blood flow at thirteen weeks was still approximately four times that of normal. In eight dogs, no significant difference was found in the 85Sr extraction by the fractured tibia and by the control, indicating that the avidity for 85Sr on the fractured side was not significantly increased. This was also true at twelve weeks in a delayed union. No consistent changes were observed its the arterial-venous differences in oxygen content, pO2, pCO2, or pH when blood from the fractured tibia was compared with that from the control side. Oxygen consumption and CO2 production, however, must have increase because blood flow was invariably increased.