Changes in the cellularity and in the nature of the matrix were studied in the cartilages of the tibial plateau in experimentally induced arthritis in the dog, 7 and 14 days after section of the anterior cruciate ligament. Samples from the different regions of these cartilages were chilled and sectioned in a cryostat, with a variable microtome chuck to allow precise orientation of the specimens. The samples were examined by normal light microscopy, by microscopic interferometry, and by quantitative polarized light microscopy. The orientation of the glycosaminoglycans was assessed by the new "induced birefringence" method. The results indicated that only the region of the medial tibial cartilage that was unprotected by the meniscus was affected, showing increased water content, loss of superficial cells, and a decrease in orientation of the glycosaminoglycans. Whereas the birefringence of the collagen was unaffected, the superficial area that lacked oriented glycosaminoglycans was markedly increased; this may be a useful indicator of early osteoarthritic changes.