Peroxisome proliferator-activated receptor γ (PPARγ) is transcription factors belong to the nuclear receptor gene family and binds to specific response elements termed PPRE, thus regulating the expression of target genes. PPARγ triggers adipocyte differentiation in adipose tissue and inhibits the cytokine production in macrophage. We analyzed the expression of PPARγ in human synovial tissues of patient with rheumatoid arthritis (RA) or osteoarthritis (OA) . PPARγ mRNA strongly expressed in synovial tissues with active RA synovitis more than that of OA. Immunohistological analysis demonstrated PPARγ localized in nuclear area of synovial lining macrophages and vascular endothelial cells in RA synovial tissues. PPARγ expression correlated with the rate of macrophage infiltration and hypervas-cularization in synovial tissues. We measured the expression of PPARγ mRNA in rat arthritic synovial tissues. PT-PCR analysis demonstrated the strong expression of PPARy in rat arthritic joints.<BR>PPARγ ligand, antidiabetic thiazolidinediones (troglitazone) inhibited the synovitis of adjuvant arthritis. These data suggest PPARγ ligands may have therapeutic application in inflammatory arthritis, such as RA.