Serum, synovial fluid and synovial tissue was taken from a total of 25 patients of both sexes on whom a synovectomy had been performed, after a 1, 3, 5, 6 or 7 day treatment with a daily administration of 20 mg Piroxicam (5 patients in each group) and the concentration of Piroxicam (Felden) determined. Parallel to this a part of the synovial tissue taken was histologically examined and classified. The analytical results presented here show that in the mean appr. 33% of the Piroxicam passed from the serum into the synovia, appr. 26% into the non or partially florid synovial tissue and appr. 32% into the florid or highly florid synovial tissue. Felden 20 (Piroxicam) thus fulfills the requirement of a good acting, non-steroidal, antiinflammatory substance (NSAID), namely of being available at the location of the inflammatory process.