The response of the rabbit knee joint to a brief episode of cytokine induced damage is described. After three intra-articular injections of catabolin/interleukin-1 all joint cartilages showed an immediate extensive loss of proteoglycan (glycosaminoglycan), which was gradually replaced over three to four weeks. Glycosaminoglycan biosynthesis (measured by 35SO4 uptake) was initially depressed, but at one week had almost doubled its rate as compared with the normal side. This increased synthetic activity was further maintained throughout the duration of the experiment (28 days), though the rate gradually fell. Histological cartilage metachromasia to toluidine blue mirrored the glycosaminoglycan changes. No disturbance of the articular cartilage collagen network was found. It is considered, therefore, that during treatment for arthritis the indigenous chondrocyte must continue to be capable of carrying out regenerative matrix repair and that antiarthritic agents should first be screened for interference with that process.