Male Dunkin Hartley guinea-pigs have been shown to develop spontaneous osteoarthritis, in the knee joint, which is histologically apparent from the age of 3 months onwards. We have used capillary electrophoresis (CE) to examine subtle changes in the sulfation of chondroitin in guinea-pig articular and xiphisternal cartilage. An age-related increase in the 6:4 sulfation ratio of chondroitin was observed, with the medial tibial plateau developing more rapidly than the other joint surfaces. The observed sulfation patterns may be related to the onset of spontaneous osteoarthritis which, in the guinea-pig, develops initially on the medial tibial plateau. Chondroitin from patellar groove cartilage of animals between 3 and 12 months of age always had a significantly lower 6:4 sulfate ratio than the other joint surfaces.