mt3mmp mmp16 is downregulated by in vitro cytokine stimulation of cartilage but unaltered in naturally occurring equine osteoarthritis and osteochondrosis matrix degradation by metalloproteinases is considered a key feature in the loss of articular cartilage seen in many joint diseases membranetype matrix metalloproteinase3 mt3mmp expression is elevated in human cartilage in endstage osteoarthritis we investigated whether mt3mmp is similarly regulated in cartilage in two naturally occurring arthropathies in vivo and whether proinflammatory cytokines regulate its expression in vitro mt3mmp expression was evaluated in cartilage from horses with osteoarthritis and osteochondrosis and compared with age and sitematched normal cartilage mt3mmp also was measured in normal cartilage stimulated with proinflammatory cytokines mt3mmp expression was not significantly altered in either osteoarthritis or osteochondrosis cartilage however gene expression was significantly downregulated by the addition of recombinant human interleukin1beta oncostatin m or tumor necrosis factoralpha to normal cartilage explants the results suggest that mt3mmp may not have a role in matrix destruction in equine cartilage diseases further work is required to characterize its regulation and function