The aim of the study was to investigate the effects of TGP, an active compound extracted from the roots of Paeonia lactiflora Pall, on the activities of synoviocytes in rats with collagen-induced arthritis (CIA) and its possible mechanisms. CIA was induced in male Sprague-Dawley (SD) rats immunized with chicken type II collagen (CII) in Freund's complete adjuvant (FCA). Synoviocytes proliferation was determined by 3-(4, 5-2dimethylthiazal-2yl) 2, 5-diphenyltetrazoliumbromide (MTT) assay. Tumor necrosis factor alpha (TNF-alpha), interleukin-1 (IL-1), prostaglandin E(2) (PGE(2)) and cyclic adenosine monophosphate (cAMP) levels in synoviocytes were measured by radioimmunoassay (RIA). E-prostanoid (EP)(2) and EP(4) receptors were analyzed by Western blot analysis. The results showed that TGP significantly inhibited the proliferation of synoviocytes, decreased the production of IL-1, TNF-alpha and PGE(2) and elevated the levels of cAMP. Further study showed that TGP could up-regulate the expression of EP(2) and EP(4). These results indicated that TGP might exert its anti-inflammatory effects through inhibiting the production of pro-inflammatory mediators in synoviocytes of CIA rats, which might be associated with its ability to regulate cAMP-dependent EP(2)/EP(4)-mediated pathway.