The levels and types of proteoglycans in articular cartilage of the knees of young beagle dogs were studied after 15 weeks of running exercise, at 4 km/day. Running increased the levels of proteoglycans in the cartilage of the patella, the superior patellofemoral groove, and the summit of the medial condyle of the femur, all of which are considered contact sites subject to enhanced loading caused by running. The elevated content of uronic acid at the femoral sites proved to be due to proteoglycans that were unable to aggregate with hyaluronic acid. There was no change in the content of aggregating proteoglycans. Analysis of chondroitinase AC-derived disaccharides at the same sites showed an increase in chondroitin-6-sulfate content as compared with chondroitin-4-sulfate levels. We believe that this modulation of the proteoglycan matrix reflects enhanced tissue maturation and physiologic adjustment to higher local contact pressures.