The effect of joint motion on one model of experimental articular inflammation, the acute arthritis produced by injection of urate crystals into canine knees, has been examined. Each increased amount of joint motion produced larger effusions, higher leukocyte counts, more severe histologic evidence of polymorphonuclear leukocyte infiltration and increased leakage of intravenously injected carbon from synovial vessels. Clearance of xenon from the joint was markedly accentuated by joint motion.Resting the inflamed joints is considered by most clinicians to be a fundamental part of the management of patients with acute arthritis (1 -5). This therapeutic approach, however, is based mostly on clinical experience rather than on controlled studies. To determine the effect that joint motion has on one model of experimental articular inflammation, the inflammatory response to a standard dose of urate crystals injected into canine knees was measured under different conditions of rest and exercise.