Histological and electron microscopic studies were performed to demonstrate the changes in the morphology of the growth plate that occur in allografts obtained from the limbs of growing rats. A genetically defined model was used in which the right hindlimbs of Lewis rats were orthotopically transplanted into Fischer-344 recipient rats. These strains are matched for major histocompatibility antigens but mismatched for minor histocompatibility antigens. The disparity at the minor histocompatibility complex between the Lewis donors and the Fischer recipients creates a weak histocompatibility barrier to transplantation. Lewis-to-Lewis syngeneic limb grafts were used as controls. The proximal parts of the transplanted tibiae were excised during acute rejection of the allograft on days 1, 5, 8, 11, 14, and 28 postoperatively. During rejection, a widened zone of calcified cartilage in the growth plate was observed at eleven days; this zone increased progressively thereafter. The number of chondroclasts in the primary spongiosa of the metaphysis had decreased significantly at eleven days, and chondroclasts had disappeared completely at fourteen days, in association with mononuclear cell infiltration. Electron microscopic examination revealed inactive morphology in some chondroclasts at eight days, and the number of inactive chondroclasts had increased significantly on day 11. At fourteen days, there were no viable chondroclasts in the primary spongiosa, and only remnants of degenerated chondroclasts were present. These findings suggest that the chondroclasts were early targets of rejection and their loss resulted in the cessation of resorption of the calcified cartilage. However, the proliferation and maturation of chondrocytes in the growth plate and the calcification of the matrix continued, despite progression of rejection in the metaphysis. Thus, survival of the chondrocytes and rejection of the chondroclasts apparently led to the formation of a widened calcifying zone in the growth plate.