To determine the effects of resistance versus aerobic exercise on the mass, strength and turnover of bone. thirty Sprague Dawley rats (4 weeks of age) were assigned to one of three experimental groups: sedentary, running or jumping. In the jumping group, the trunk was kept upright during electrically stimulated jumping exercise for 1 h every other day. The running rats ran at speeds of 24 m/min for 1 h every other day. After 4 weeks, the jumping rats exhibited increases in the mass and strength of the lumbar vertebrae and of the mid-diaphysis of the femur (mid-femur), and increases in the cross-sectional morphology of these bones: the trabecular bone volume per bone surface, the trabecular thickness, the trabecular bone formation rate per bone surface (BFR/BS). In addition, they exhibited reduced trabecular separation and the area of osteoclast surface per bone surface. The running and sedentary rats showed no such changes. With regard to the mid-femur, in both the jumping and running rats the periosteal BFR/BS was increased. However, only the jumping rats showed a reduction in the BFR/BS at the endocortical surface. These results suggest that resistance exercise accelerates cortical drift and increases the bone mass and strength by stimulating bone formation more efficiently than does aerobic exercise.