Patients with rheumatoid arthritis were medicated with 4 g. benorylate twice daily for periods of 1–24 days prior to therapeutic synovectomy. The synovial fluid and excised synovial tissue were then examined and the intact benorylate molecule was found in 10 out of 18 patients at concentrations ranging from 0.9–5.2 μg./g. tissue. This concentration appeared to be dependent on the duration of medication.