Summary Objective We previously reported that nonsteroidal anti-inflammatory drugs (NSAIDs) aggravated cartilage degeneration and that hyaluronate (HA) reversed the cartilage destruction accelerated by NSAIDs in a rabbit osteoarthritis (OA) model. In the present study, we analyzed the mechanisms of these effects of NSAIDs and HA. Method An experimental rabbit model of OA was induced by partial meniscectomy and treated with NSAIDs, HA and the combination of NSAIDs and HA over 2 weeks. After sacrifice, cartilage degeneration and the levels of matrix metalloproteases (MMP)-1, MMP-3 and MMP-13 in synovial lavage fluid were measured. In an in vitro study, human articular chondrocytes were cultured with NSAIDs and HA in the presence of interleukin (IL)-1β or IL-6 + sIL-6 for 24 h. After culture, the production of MMPs, IL-1β, and IL-6 was measured. Results The levels of MMP-1, MMP-3 and MMP-13 in synovial fluid from the NSAID-treated group were significantly higher than in controls. The increased production of MMPs induced by NSAIDs was counteracted by the concomitant administration of HA. In vitro studies showed that NSAIDs augmented IL-1β- and IL-6-induced production of MMPs from human chondrocytes, while completely inhibiting the IL-1β- and IL-6/sIL-6R-induced production of prostaglandin E2 (PGE2 ). The addition of PGE2 did not reduce the augmentation of MMP production. HA inhibited the promotion of cytokine-induced MMP and cytokine production by NSAIDs. Conclusions Our data clearly showed that HA inhibited NSAID-accelerated MMP production which was followed by inflammatory cytokine production from cytokine-activated chondrocytes. The results warrant further evaluation of the potential chondroprotective effects of co-administration of HA with NSAIDs.