1. Venous occlusion plethysmography has been used to measure the blood flow in the calves of nineteen patients with fractures of the lower limb and in six normal control subjects.
2. The resting blood flows were significantly higher in the injured legs than in uninjured legs, irrespective of the site of injury. Flows in the uninjured limbs were similar to those of the control subjects.
3. During reactive hyperaemia after 10 min arterial occlusion, the increase of flows in both legs of the patients was significantly lower than in the control subjects. Because of the increased resting flow, the maximal flow in the injured leg was similar to that in the control subjects, whereas the maximal flow in the uninjured leg was significantly lower than in the control group.
4. The changes in resting flow cannot be accounted for by a change in the proportions of tissues in the limb but they may be explained by an increase of the flow through muscle secondary to a relative increase in the mass of slow to fast muscle fibres.