A study was made of the extent of the intracortical vascular damage occurring on intramedullary reaming in the rabbit tibia with simultaneous reduction of pressure in the distal part of the medullary cavity, and the findings were compared with previously reported results from rabbits in which reaming was performed without such pressure reduction. It was found that reduction of pressure, on reaming, by suction through a separate, peripheral opening in the distal metaphysis, considerably diminished the vascular damage in the cortex. In these cases the primarly avascular part of the cortex began to be revascularized two weeks after the operation, a process which was almost completed after a further two weeks. When the pressure in the medullary cavity was not reduced during the reaming, the degree of intracortical avascularity was considerably greater, and persisted to a large part after an observation time of four weeks.