Pre- and poststudy motion and gait analyses of eight size-matched male greyhounds confirmed uniform loading of their femora. Subminiature strain gages implanted on the intact inferior and anterior aspects of the femoral neck in six greyhounds indicated in vivo strain variations among test animals. Motion and gait analyses confirmed uniform loading of femora following unilateral hemiarthroplasty with cobalt-chromium hip implants. In vivo strain measurements adjacent to the implants indicated large variations among test animals. A consistent direction of strain change relative to the intact femur was noted, even though strain changes varied in magnitude. Image analysis of microradiographs indicated insignificant differences in the cortical areas of implanted and intact femora. Extensive new trabecular bone formation was noted along the implant in the endosteal cavity and correlated with a combination of implant placement and exercise level. Most of the bone was formed with centrally placed implants in exercised dogs, and the least with stems on the medial neck surface in rested dogs. Iliac crest biopsies indicated that bone formation rates slowed in rested animals and remained constant throughout the study in exercised animals. All implanted femora had a thin (<1 mm thick) aligned fibrous tissue layer separating the implant from bone. It varied in thickness as a function of the aspect of the implant. Exercised dogs had a larger proportion of fibrous tissue on the anterior and posterior aspects, while rested dogs had a larger proportion of fibrous tissue on the medial and lateral aspects.