OBJECTIVE To determine effects of transforming growth factor (TGF)-β and interleukin (IL)-1β on inflammatory markers in cultured canine chondrocytes to clarify the role of these cytokines in osteoarthritis of dogs. SAMPLE Pooled chondrocytes isolated from the stifle joints of 4 adult dogs. PROCEDURES Chondrocytes were isolated, cultured, and frozen at -80°C. Pooled cells were incubated in medium with or without TGF-β (1 or 10 ng/mL) and subsequently stimulated with IL-1β (10 ng/mL). Concentrations of nitric oxide (NO) and prostaglandin (PG) E were measured in culture supernatants. Gene expression of matrix metalloproteinase (MMP)-3, tissue inhibitor of metalloproteinase (TIMP)-2, inducible NO synthase (iNOS), and cyclooxygenase (COX)-2 was quantified by use of real-time quantitative PCR assays. RESULTS Stimulation with IL-1β increased gene expression of all inflammatory markers, except for TIMP-2. Incubation with TGF-β resulted in a significant decrease in MMP-3 and TIMP-2 mRNA concentrations but had no effect on PGE and NO concentrations. For cells treated with TGF-β followed by IL-1β, concentrations of PGE and NO were lower, compared with concentrations for IL-1β control cells. Furthermore, IL-1β-induced gene expression of iNOS, MMP-3, and COX-2 was downregulated. However, the IL-1β-induced downregulation of TIMP-2 gene expression was partially restored by pretreatment with TGF-β. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that IL-1β increased the expression of inflammatory genes and mediators, and TGF-β largely attenuated the IL-1β-mediated inflammatory response. Therefore, TGF-β might be a novel target for use in the prevention and treatment of cartilage breakdown in dogs with osteoarthritis.