The tissue response of rat tibiae to the surgical placement of commercially pure titanium implants was examined at 2, 6, 10, and 28 days. The transcortical placement of 1.5-mm x 2-mm implants resulted in the apposition of threaded implant surfaces within cortical and cancellous regions of the tibia. In all regions, evidence of bone formation was obtained through pre-embedding fracture of the implant from the bone tissue interface. Scanning electron microscopy examination of early responses revealed a fibrin clot and rapid formation of a loosely organized collagenous matrix. Many extravasated blood cells contacted the implant surface. At day 6, a more organized matrix containing many blood vessels opposed the implant surfaces, and few extravasated blood cells remained in contact with the implant surface. By day 10, the surgical wound was filled with woven bone that approximated the contours of the threaded implant. Later, few cells were attached to the retrieved implants. The consolidation of the forming matrix was clearly evident at 28 days. The tissue interface was an amorphous metrix that revealed the surface characteristics of the machined implant. Ligh microscopic analysis of ground sections indicated that, from day 6 onward, cells morphologically consistent with the osteoblastic phenotype were predominant within the gap between the surgical margin and implant surface. Osteoblastic cells had achieved the formation of on osteoid seam upon which bone form ation progressed. The amtrix that had formed represented woven bone containing many osteocytes. At day 6, evidence of remodeling was observed at sites distant from t he surgical site, and by day 28 osteoclastic activity was observed at trabecular sites adjacent to the impl ant surface. The rat tibia model provides evidence of rapid formation of bone at implant surfaces.