The nearly constant finding of sodium urate crystals in aspirated gouty synovial fluid (1) led to the induction of acute arthritis in both human (2, 3) and canine (3) subiects by intrasynovial crystal injection. Calcium pyrophosphate crystals, identified in the synovial fluid from patients with a goutlike syndrome ("pseudogout"), and crystals of adrenocorticosteroid esters are also irritants when injected (4, 5). Crystal-induced inflammation is completely reversible, dose related, and nonspecific with regard to host or to the chemical composition of the injected crystal. These observations confirmed and extended findings recorded by His and Freudweiler at the end of the nineteenth century (6)(7)(8)).An experimental model of crystal synovitis in the dog and several methods for its quantification are described herein. Applications of these methods to the study of the host response mechanism are described in the accompanying paper (9) and in reference 10. MethodsGeneral Considerations.--The knee (stifle) joints of mongrel dogs weighing 12 to 25 kg were used. Light anesthesia was induced with intravenous sodium pentobarbital (25 to 35 mg/kg); additional small doses were given as needed for maintenance. One experiment was performed using the "physiologic" anesthetic chlorolose (50 mg/kg intravenously). Body temperature, monitored by rectal thermometer, was controlled with an electric blanket within a range of 2°C. In some experiments, the trachea was intubated and breathing was maintained with a me-*

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