Phenylbutazone was measured in the synovial fluid of 15 patients and found to be present in appreciable amounts which were related to plasma levels. 2. 80% of fluid levels were between 55% and 80% of those in plasma. 3. There was some evidence that in very actively inflamed joints phenylbutazone levels were lower. 4. There was no relationship to plasma or synovial fluid protein levels. 5. Phenylbutazone was found in one patient's synovial fluid three weeks after stopping the drug.
