Little is known of the effects of synovectomy on articular cartilage. In order to investigate this matter, anterior synovectomy of the knee was performed in thirty-five normal adult rabbits and in thirty-five which were given 25 milligrams of hydrocortisone intramuscularly each week afterwards. The animals were killed at intervals from four to 110 days after synovectomy. Histological examination of the regenerating synovium in both groups showed complete structural and functional regeneration by eighty days in the first group and a delay in regeneration in the steroid group. ³⁵Sulphur autoradiographs of the articular cartilage of femoral and tibial condyles revealed surface fibrillation and chondrocyte death in 23 per cent of normal knees after eighty days but only 1·8 per cent of knees of animals receiving hydrocortisone. Thus synovectomy in a healthy joint may have an unfavourable effect on the physiology of cartilage by alteration of synovial composition and hyaluronate content in normal joints. Systemically-administered hydrocortisone may reduce this harmful effect in normal cartilage.