Cylindrical porous-coated implants were placed in the distal femoral metaphyses of twenty dogs and were subjected to zero, twenty, forty, or 150 micrometers of oscillatory motion for eight hours each day for six weeks with use of a specially designed loading apparatus. The in vivo skeletal responses to the different magnitudes of relative motion were evaluated. Histological analysis demonstrated growth of bone into the porous coatings of all of the implants, including those that had been subjected to 150 micrometers of motion. However, the ingrown bone was in continuity with the surrounding bone only in the groups of implants that had not been subjected to motion or that had been subjected to twenty micrometers of motion; in contrast, the implants that had been subjected to forty micrometers of motion were surrounded in part by trabecular bone but also in part by fibrocartilage and fibrous tissue, and those that had been subjected to 150 micrometers of motion were surrounded by dense fibrous tissue. Trabecular microfractures were identified around three of the five implants that had been subjected to forty micrometers of motion and around four of the five that had been subjected to 150 micrometers of motion, suggesting that the ingrown bone had failed at the interface because of the large movements. The architecture of the surrounding trabecular bone also was altered by the micromotion of the implant. The implants that had stable ingrowth of bone were surrounded by a zone of trabecular atrophy, whereas those that had unstable ingrowth of bone were surrounded by a zone of trabecular hypertrophy. The trabeculae surrounding the fibrocartilage or fibrous tissue that had formed around the implants that had been subjected to forty or 150 micrometers of motion had been organized into a shell of dense bone tangential to the implant (that is, a neocortex outside the non-osseous tissue).