Objective. To evaluate the role of the MEK/ERK MAP kinase pathway in murine collagen-induced arthritis (CIA) using the selective MEK inhibitor PD184352. We examined the effects of the inhibitor in cytokine-stimulated synovial fibroblasts and in cytokine-induced arthritis in rabbits to investigate its antiinflammatory mechanisms. Methods. Murine CIA was used to assess the effects of the selective MEK inhibitor on paw edema, clinical scores, weight loss, histopathologic features, and joint levels of p-ERK. Western blotting and immunohistochemistry techniques were used to assess p-ERK in human and rabbit synovial fibroblasts and synovial tissue from rheumatoid arthritis (RA) patients. Interleukin-1α (IL-1α)-stimulated stromelysin production in rabbit synovial fibroblasts was assessed by enzyme-linked immunosorbent assay. A rabbit IL-1α-induced arthritis model was used to assess the effects of the inhibitor on IL-1α-induced MEK activity, stromelysin production, and cartilage degradation. Results. In the CIA model, PD184352 inhibited paw edema and clinical arthritis scores in a dose-dependent manner. Disease-induced weight loss and histopathologic changes were also significantly improved by treatment. Inhibition of disease-induced p-ERK levels in the joints was seen with the inhibitor. Levels of p-ERK in the synovium were higher in RA patients than in normal individuals. PD184352 reduced IL-1α-induced p-ERK levels in human RA synovial fibroblasts. The production of p-ERK and stromelysin was also inhibited in IL-1α-stimulated rabbit synovial fibroblasts. We observed IL-1α-induced p-ERK in the synovial lining, subsynovial vasculature, and articular chondrocytes. IL-1α-induced stromelysin production and proteoglycan loss from the articular cartilage were reduced by PD184352. Conclusion. These data demonstrate the inhibition of murine CIA by PD184352, support the hypothesis that antiinflammatory activity contributes to the mechanism of action of the inhibitor, and suggest that a selective inhibitor may effectively treat RA and other inflammatory disorders.