Nitric oxide (NO) plays an important role in cartilage degeneration, and NO donors induce meniscus degeneration and synovium inflammation. This study evaluated the effect of intraarticular injections of hyaluronan (HA) on NO production in meniscus and synovium using an experimental osteoarthritis (OA) model. Thirty-six New Zealand white rabbits underwent unilateral anterior cruciate ligament transection (ACLT), and were divided into three groups. Four weeks after ACLT, the HA group started to receive intraarticular HA injections once a week for 5 weeks; the vehicle group started to receive the carrier of HA; and the no injection group, no treatment. All ACLT knees were harvested at the 9th week. Meniscus and synovium sections were examined by immunohistochemistry for nitrotyrosine. The pieces of these two tissues were cultured for 24 h. Culture supernatants were analyzed for nitrite concentration. The amount of NO produced by the meniscus was much larger than that produced by the synovium. NO productions in the meniscus and synovium of the HA group were significantly lower than those of the other groups. The results suggest that the inhibition of NO production in meniscus and synovium might be a part of the mechanism of the therapeutic effect of HA on OA.