Effect of eleven non-steroidal anti-inflammatory drugs on the acyl-CoA synthetase activities toward octanoic, palmitic, arachidonic and docosahexaenoic acids was evaluated in mouse liver and brain mitochondria. The drugs tested were aspirin, salicylic acid, diflunisal, mefenamic acid, indomethacin, etodolac, ibuprofen, ketoprofen, naproxen, loxoprofen, flurbiprofen. In mouse liver mitochondria, diflunisal and mefenamic acid exhibited the inhibitory activities not only for octanoic acid (IC(50) = 78.7 and 64.7 µM) and but also for palmitic acid (IC(50) = 236.5 and 284.4 µM), respectively. Aspirin was an inhibitor for the activation of octanoic acid only (IC(50) = 411.0 µM). In the brain, mefenamic acid and diflunisal inhibited strongly palmitoyl-CoA formation (IC(50) = 57.3 and 114.0 µM), respectively. The activation of docosahexaenoic acid in brain was sensitive to inhibition by diflunisal and mefenamic acid compared with liver.