Little is known of the physiology of the synovial membranes, particularly regarding their permeability. As a consequence, it is difficult to understand the effusions occurring in joint cavities, or the accumulations of fluid in tendon sheaths and in bursae, consequent to trauma or disease. The knee joint is usually studied since it is easily accessible. In addition, it possesses the most extensive synovial membrane in the body. Diffusible constituents pass readily from the blood into the synovial fluid. In fact Crouter, Cajori and Pemberton1 have shown that their concentration in synovial fluid can be readily changed by inducing corresponding changes in the blood. Fisher2 observed that potassium iodide, Berlin Blue, and colloidal silver were absorbed from the knee joints of rabbits. He demonstrated iodide in the urine but did not determine in this manner its rate of absorption from the synovial cavity.Using the method of Dandy and Rowntree,3 we have studied the permeability of the synovial membrane to phe...