Reconstruction of the anterior cruciate ligament (ACL) with patellar tendon (PT) is a common procedure for the symptomatic ACL-deficient knee. Questions regarding graft incorporation, viability, and nutrition of the transplanted tissue are of concern. This relates to the graft's response to its new intrasynovial milieu and new physical forces. These factors were studied in a rabbit model of ACL reconstruction using PT and were evaluated with histological and biochemical parameters with respect to time. A histological and biochemical metamorphosis of the grafted PT occurred in this study. Autografts demonstrated a gradual assumption of the microscopic properties of normal ACL; by 30 weeks postoperatively, cell morphology was ligamentous in appearance. Normally, type III collagen is not observed in PT, however, a gradual increase in its concentration was seen in the grafts; by 30 weeks its concentration (10%) was the same as in normal ACL. Similarly, glycosaminoglycans content increased from its normally low level in PT to that found in native ACL. Collagen-reducible crosslink analysis demonstrated that grafted tissue changed from the normal PT pattern of low dihydroxylysinonorleucine (DHLNL) and high histidinohydroxymerodesmosine (HHMD) to the pattern seen in normal ACL (high DHLNL and low HHMD) by 30 weeks. These data suggest that when PT is placed in the anatomic and environmental milieu of the ACL, a "ligamentization" of the grafted tissue results; also the autograft initially depends on synovial fluid nutrition, as revascularization occurs after 6 weeks.