Abstract.The effect of total body irradiation on bone resorption is studied. Resorption is induced by applying an orthodontic device. A greater increase in bone resorption is observed in animals submitted to total body irradiation plus orthodontic force application, than in those submitted to orthodontic force alone. The increase in erosive areas began before an increase in the number of osteoclasts was noted. Irradiation resulted in an increased number of osteoclasts.