Fenamates are non-steroidal anti-inflammatory agents (NSAIDs). Flufenamic acid (FFA) activates TRPA1 and TRPC6, but inhibits TRPM2/4/5, TRPC3 and TRPC7 (Eisfeld and Lückhoff, 2007; Hu et al., 2010). However, the effect of fenamates on TRPC4 and TRPC5 is still unknown. To understand the structure-activity of fenamic compounds on TRPC channels, we synthesised a series of fenamic analogues and investigated their effect on TRPC4 and TRPC5 channels. Human TRPC4 and TRPC5 cDNAs in tetracycline-regulated (tet-on) pcDNA4/TO vectors were stably transfected into T-REx HEK293 cells. The expression was induced by 1 µM tetracycline and the whole cell current of TRPC4 or TRPC5 was recorded by patch clamp. Calcium imaging was used for the measurement of Ca2+ influx and Ca2+ release in the cells. We found that FFA, mefenamic acid (MFA), niflumic acid (NFA) and diclofenac sodium (DFS) inhibited TRPC4 and TRPC5 channels in concentration-dependent manner. The potency was FFA>MFA>NFA>DFS. Modification of the 2-phenylamino ring by substituting the three fluorine atoms in FFA with -F, -CH3, -OCH3, -OCH2CH3, -COOH and -NO2 led to changes in the blocking activity. Selective COX1-3 inhibitors (aspirin, celecoxib, acetaminophen and indomethacin) had no effect on the channel activity. However, longer perfusion (>5 min) with FFA (100 µM) and MFA (100 µM) caused a gradual increase in TRPC4 and TRPC5 currents after their initial inhibitory effects. Our results suggest that fenamates and their new analogues are TRPC4 and TRPC5 channel blockers. The polarity and confirmation of the 2-phenylamino ring could alter their blocking activity on the channels. No effect of non-fenamate inhibitors on COX1-3 indicated that the channel blocking effect is not mediated by the COX inhibition. These findings give a clue that the direct effect on TRPC channel activity or Ca2+ influx could be one of the mechanisms of fenamates.