The effect of diclofenac sodium (DS) on the metabolism of glycosaminoglycans (GAG) was studied in rabbits during the development of osteoarthritis of the knee induced by immobilization. Contents of hexosamines, uronic acid and sulphate-derived 35S-radioactivity in separated GAGs were determined. DS was given to 6 immobilized rabbits for 17 days at a dose of 1.5 mg/kg through a stomach tube. The controls consisted of 8 immobilized rabbits without medication and of 21 non-immobilized rabbits, 6 of which received DS for 17 days. DS did not inhibit in vivo metabolism of sulphated GAGs, according to measurements of [35S]-sulphate incorporation. Healthy rabbits given DS showed only marginal changes of GAG content in joint tissues. DS did not prevent the loss of GAG from the weight-bearing cartilages of the immobilized knees, but caused a further loss of chondroitin sulphates accompanied by an increased amount of hyaluronic acid in the tissues. DS prevented accumulation of the GAGs, which normally occurs during development of osteoarthritis in tissues of the tibial margin and in collateral ligaments. The findings indicate that the effect of DS on the GAG metabolism in connective tissues is somewhat different from that with other non-steroidal anti-inflammatory agents.