The anti-inflammatory effect of propolis was compared with that of diclofenac, a non-steroidal anti-inflammatory drug, and L-N(G)-nitro arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, using carrageenin-induced mouse paw edema. When administered 10 min prior to carrageenin injection, propolis (1 : 1000, 1 : 100, p.o.), diclofenac (12.5, 50 mg/kg, p.o.) and L-NAME (10, 100 mg/kg, s.c.) showed a significant anti-inflammatory effect. The anti-inflammatory effects of propolis and L-NAME were significantly inhibited by L-arginine, a precursor of nitric oxide, but not by D-arginine. In contrast, the anti-inflammatory effect produced by diclofenac was not inhibited by either D-arginine or L-arginine. These results indicate that the anti-inflammatory effect of propolis on mouse paw edema acts via the inhibition of nitric oxide production, similar to that of L-NAME but not diclofenac.