Levels of tissue inhibitor of metalloproteases (TIMP) and plasminogen activator (PA)/plasmin were measured and the distribution of PA was studied by immunohistochemical techniques in cartilage and synovium samples from dogs subjected to sectioning of the anterior cruciate ligament of their right knees and sham operation of their left knees (controls). Twenty-three animals were divided into 3 groups and killed at 2, 4, or 8 weeks after surgery. The levels of PA and plasmin were found to be significantly elevated in the osteoarthritic (OA) knee cartilage and synovium at all times after surgery, except for levels of PA in the OA cartilage at 2 weeks. There was a positive correlation between the levels of PA and plasmin in the synovial membrane (r = 0.64, P less than 0.001). In OA knees, the presence of high levels of total and active collagenase was detected in cartilage and in synovium. The levels of these 2 forms of collagenase showed a positive correlation both in cartilage (r = 0.65, P less than 0.001) and in synovium (r = 0.77, P less than 0.001). The levels of TIMP in cartilage from OA and sham operated knees were similar. Although the TIMP level was increased in the OA synovium, it was found only in trace amounts in cartilage. Immunohistochemical studies revealed that both forms of PA, urokinase-type PA and tissue-type PA, and TIMP were present in OA tissues. In the synovium, they were found mainly in monocyte/macrophages, synovial lining cells, and blood vessel cells. In OA cartilage, PA was present only at the superficial level in chondrocytes and in cartilage matrix, whereas TIMP was present in chondrocyte lacunae throughout the full thickness of the cartilage. TIMP was also detected in the superficial level of cartilage from sham operated knees. The results of this study indicate that in OA tissues, there are conditions that favor the synthesis and activation of metalloproteases. PA and plasmin are likely to play an important role in the physiologic activation of metalloproteases, although they are probably not the only system involved in this process. The lack of increased TIMP levels in the OA cartilage, in the presence of increased metalloprotease activity, is also a possible contributing factor in the enzymatic degradation of this tissue.