Intra-articular injection of hyaluronate (HA) is widely used in the treatment of arthropathies. However, the mechanism of the effects of HA preparations on the arthritic synovium and the relationship between their effects and molecular weights (MW) remains unknown. The objectives of this study were to compare the effects of two hyaluronate preparations, HA84 (MW: 84×10⁴) and HA230 (MW: 230×10⁴), on the synovium of an arthritis model and to examine the mechanism of the effects of HA. The HA preparations were intra-articularly injected in a model of canine arthritis induced by anterior cruciate ligament transection for a total trial of 5 weeks. To define the accessibility of HA preparations to the synovial lining layers, fluorescein-labeled HA84 or HA230 was injected at the last administration. Pathological changes analyzed included increases in volumes and prostaglandin E₂ concentrations in synovial fluids, thickening of the synovial lining layers, vacuolar alterations in the lining cells, and stainability of HA in the synovium. Expression levels of Heat shock protein 72 (Hsp72) were immunohistochemically detected in the tissues to investigate the ability of the cells to survive the degeneration. The pathological changes described above were more significantly suppressed in the HA84-treated than in the HA230-treated groups. In most cases of the HA84-treated group (five cases out of six), fluorescein particles were intensely distributed in the synovial lining layers, but only two cases in the HA230-treated group showed a weak distribution of fluorescein particles in the layers, indicating a certain differnce in the accessibility of HA preparations to the lining cells between the two HA molecules. Moreover, the immunoreactivity for Hsp72 in the lining cells as more intense in the HA84-treated than in the HA230-treated groups. The difference in the accessibility of HA molecules corresponded well with that in the inducibility of Hsp72 in the lining cells. These results suggest that the up-regulation of Hsp72 may offer a new concept concerning mechanism of the effects of HA preparations on the arthritic Synovium.