1. Protein, chloride, sugar and non-protein nitrogen have been determined in plasma and pathological synovial fluids in twenty-three instances.

2. The protein content of the synovial fluids is less than that of the plasma. The chloride content is greater than that of the plasma. This inverse relationship of protein and chloride is analagous to that found between plasma and peritoneal effusions, pleural effusions, and the cerebrospinal fluid, and is probably influenced by the Donnan Membrane Equilibrium.

3. Low plasma chloride is accompanied by low chloride in the synovial fluid.

4. The non-protein nitrogen is approximately equally distributed between plasma and synovial fluid.

5. In fasting patients, the sugar content of non-infected synovial fluid is usually slightly lower than that of the plasma.

6. The hyperglycemia caused by anaesthesia is accompanied by a rise in sugar content of the synovial fluid.

7. In four instances of bacterially infected fluids the sugar content was markedly lowered, while in two cases of tuberculosis of the joint the sugar content was moderately lowered. This is analagous to the low sugar content of the cerebro-spinal fluid in purulent and tuberculous meningitis.

8. It is suggested that determination of the sugar content of synovial fluids may prove to be of diagnostic value.