As a sustained release preparation of diclofenac sodium (DF-Na), we developed a preparation (SR318B) that was expected to exhibit a persistent analgesic effect by once-daily oral administration. In this study, we investigated the persistence of the analgesic effect of SR318B using female cynomolgus monkeys with arthritis induced by sensitization with bovine type II collagen combined with FreundÕs complete adjuvant. Cynomolgus monkeys in which arthritis was induced to the same severity received oral administration of SR318B (1 mg DF-Na/kg) once daily for 14 days. The mean ellipsoid area of the proximal interphalangeal joints in the monkeys slightly increased before administration on day 3 in the control group. In contrast, the area decreased after day 3 and significantly decreased on day 13 in the SR318B treatment group. The plasma diclofenac (DF) level reached the highest point at 6 hr after administration on day 13, then decreased. The plasma level was lower than the quantification limit 20 hr after administration on both days 0 and 13. In contrast, DF was detected in synovial fluid 24 hr after the initial and final administration. Based on the above findings, SR318B exhibits a persistent analgesic effect on collagen-induced arthritis. While the plasma DF concentration decreased to a level lower than the quantification limit at 20 hr, DF was still detected in the synovial fluid 24 hr after administration. Therefore, the retention of DF in inflammatory regions may play an important role in the analgesic effect.