BACKGROUND: The spinal administration of neostigmine has been shown to produce analgesia, but this analgesia is limited by adverse effects. This study was designed to determine whether intra-articular neostigmine results in an analgesic effect in a rat inflamed knee joint model, and to investigate the possible involvement of nitric oxide in neostigmine-induced analgesia. METHODS: Male Sprague Dawley rats were divided into three groups; control group, neostigmine group (1, 3, 10micro gram), and L-NAME group (neostigmine 1, 3, 10micro gram + L-NAME 500micro gram). Inflammation was induced by injecting 2% carageenan into the right knee joint. Intraarticular drugs were injected at an hour after the injection of carageenan. Effects of these drugs were assessed before and at 4, 8, 12, 24, 60, and 120 hours after inflammation using a Dynamic plantar aesthesiometer. RESULTS: There were significant differences between the neostigmine 3, and 10micro gram group and the control group. There were no significant differences between the neostigmine group and the L-NAME group except for in the neostigmine 10micro gram L-NAME group at 24 hours. There were no significant differences between the neostigmine 3micro gram L-NAME group and the control group. This result shows that injection of L-NAME partially reverses the anlgesic effects of the neostigmine. CONCLUSIONS: Intra-articular neostigmine results in an analgesic effect at the site of inflammation. Although this study could not prove the involvement of nitric oxide in the peripheral analgesic effect of neostigmine, it demonstrates the possible involvement of nitric oxide in neostigmine-induced analgesia.