By measuring knee-joint cavity and subchondral bone temperatures by means of a Telethermometer thermister needle in both normal healthy subjects and rheumatoid patients, a rise in temperatures after standard exercise was demonstrated. The temperature rise was markedly greater in rheumatoid diseased joints than in healthy joints. Since the wear or degeneration of a joint depends on frictional resistance and consequent temperature rise, an inbuilt cooling mechanism in healthy joints is postulated, and it is suggested that by such temperature measurement in rheumatoid and osteo-arthritic patients, a method of objective assessment of the efficacy of anti-inflammatory drugs is available unbiased by personal judgement. © 1981.