The tissue concentration of diclofenac sodium, the active substance of Voltaren SR 75 mg tablets, was studied in 10 patients with coxarthrosis. Voltaren SR 75 mg tablets were administered twice daily for 5 days, then prosthetic joint replacement of the hip was performed. Tissue levels of diclofenac were determined by HPLC in specimens of articular cartilage, synovial membrane and bone obtained during surgery. The results demonstrate that diclofenac was present in all three tissue types after 5 days of therapy.