Transection of the canine anterior cruciate ligament is used widely as a model of osteoarthritis (OA). The results from studies using this model have sometimes differed, but various areas of the joint have been compared. In other studies, the exaggerated chondrocyte synthetic response in the medial femoral condyle in this model which leads to hypertrophic repair was characterized. In this study the magnitude of this reaction was analyzed in 8 anatomically defined areas of the joint using 28 dogs followed from 3 days to 64 weeks after transection of the anterior cruciate ligament, 8 nonoperated control animals and 7 sham operated control animals. The results show that the in vivo reaction differs markedly among different anatomical areas of the joint. This suggests that the response is modulated by differences in the biomechanics of the joint. Furthermore, it suggests caution in comparing results of studies of OA cartilage if different areas of the joint have been used.