The objective of this study was to comprehensively investigate the mRNA expression levels of proinflammatory cytokines and a growth factor (IL-1β, IL-6, IL-8, TNF-α, and TGF-β), inflammatory mediators (COX-2, prostaglandin E synthase, PGES and EP4), nerve growth factor and its receptor (NGF and TrkA), and matrix metalloproteinases (MMP-3 and MMP-13) in the synovial membranes of dogs with spontaneous OA as well as to elucidate their relationships with the severity of synovitis. The gene discussed is PTGER4; the disease is synovitis.