In patients with rheumatoid arthritis, as well as in persons with other kinds of synovitis, proteins enter the knee joint more rapidly than in normal individuals (P less than 0.001). The rheumatoid synovium, however, is less permeable to small molecules (tritiated water, P less than 0.02; urate, P less than 0.05; and glucose, P less than 0.002) than is the normal joint lining. This difference is explained if rheumatoid microvascular changes enhance synovial permeability to proteins while coexisting interstitial changes diminish synovial permeability to smaller molecules.