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The metabolic half-life of hyaluronan (HA) in synovial fluid was estimated in sheep from the rate of appearance of ³H₂O in plasma after injection of highly polymerized labeled HA. This material is substituted with ³H in its acetyl group and is rapidly and almost completely degraded in sheep and other species to yield ³H₂O. Previously sensitized sheep were studied before and after induction of acute monoarticular arthritis by intraarticular challenge with type II collagen. In both circumstances ³H was released from the joint in a monophasic exponential pattern and appeared in plasma only as ³H₂O. Before challenge, the mean metabolic half-life of [³H]HA was 20.8 hours (range, 15.8 to 27.9 hours, n = 5); an estimate in a single unsensitized sheep (27.0 hours) fell within this range. After challenge, swelling occurred around the joint without frankly increased synovial fluid. The mean half-life fell to 11.5 hours (range, 9.0 …