We have attempted to verify the suggestion that synovial membrane is the result of mechanical disruption of connective tissue, and may occur at any site. Mechanical disruption of the subcutaneous connective tissue was achieved in rats and mice by the repeated injection of air. The resulting cavities developed a lining structure with many of the features of synovial membrane as judged by electron microscopy, and light microscopy using haematoxylin and eosin and van Gieson stains, esterase activity and immunofluorescent staining for Ia antigen. A structure closely resembling synovium is formed as early as 6 days, providing a convenient method for studying large quantities of facsimile synovial tissue under a wide variety of easily administered stimuli.