Animal models for chondral shaving and subchondral abrasion were created to resolve the controversy about the nature of the repair tissue after these procedures and to determine the effect of continuous passive motion on the quality of the repair tissue. Chondral shaving was performed on the patella in forty adolescent rabbits, and subchondral abrasion was performed on the patella in another forty rabbits. In both procedures, a three-millimeter-diameter defect was created. After the operation, twenty animals from each group were allowed intermittent active motion; the remainder were treated by continuous passive motion for two weeks, followed by intermittent active motion. Half of the animals from each group were killed at four weeks and the other half, at twelve weeks. There was no evidence of repair tissue in the defects at either four or twelve weeks after chondral shaving, regardless of the postoperative treatment. The remaining underlying cartilage, however, had degenerated. After abrasion of subchondral bone, the defects in animals that were treated with only intermittent active motion healed at twelve weeks, although the quality of the repair tissue varied. All ten of the animals that were treated with continuous passive motion, however, had mature, hyaline-like cartilage as the predominant repair tissue at twelve weeks, compared with six of the ten animals that were treated with intermittent active motion (p less than 0.05). We concluded that, in this model, partial-thickness defects created by chondral shaving do not heal; rather, the remaining underlying cartilage degenerates. Full-thickness defects created by subchondral abrasion can heal by regeneration of hyaline-like cartilage. Such healing is enhanced by continuous passive motion for two weeks postoperatively.