14 male rats were divided into exercise and control groups to examine the effect of a 1-month exercise program on the vascular morphology of the tibial diaphysis. Following the exercise program the number of haversian canals per mm2 increased in the middle third of the cortex, but not in the outer or inner zone. No change was observed in the number of nonhaversian canals. The size of the nonhaversian canals increased following the training program, which was not evident in the haversian canals.