Using the microsphere technique bone blood flow was measured in different anatomical and functional regions in long bones in conscious dogs. The measurements were performed during physical exercise upon a treadmill, and the bone blood flow values were obtained as prework resting values after 1 and 2 hours of exercise and after 1 hour of rest. The perfusion rates increased 50 per cent from 1.6 to 2.5 ml X 100 g tissue-1 X min-1 in the femoral and tibial cortical bones during work. In the cancellous bone of the femoral head an increase from 12.6 to 20.6 ml X 100 g tissue-1 X min-1 was found. Equal flow responses were determined in the fat-filled tibia-condylar and femoral supracondylar bone. The increase took place after 2 hours' exercise, but nonstatistically verified increased perfusion was found after 1 hour's work. The alternation in bone blood flow suggests that bone has a capability of physical vasodilation during muscular work but the flow response is slow and therefore the vasodilatation seems mediated by a metabolically induced stimulus.