Effects of garlic on isolated rat aorta were investigated by comparing with those of acetylcholine and L-arginine in the presence and absence of endothelium. For this purpose, certain linear and non-linear regression models were applied for concentration-response curves obtained by acetylcholine, L-arginine and garlic in the rat aorta. Garlic caused dose-dependent relaxations in isolated rat aorta which were attenuated by the removal of endothelium as in the case of acetylcholine. However, the relaxant responses to acetylcholine, L-arginine and garlic were not completely abolished by the endothelial denudation. Application of a number of regression models for the vasorelaxant effects of acetylcholine and garlic revealed that mechanism(s) of the effect of garlic may be different from that of acetylcholine. Furthermore, it was noted that L-arginine-induced relaxations, but not those induced by acetylcholine and garlic, are enhanced by a 2 or 4 h incubation period in the bathing medium. The findings obtained strongly suggested that the vasorelaxant effect of garlic is important in its hypotensive activity and mediated by the production of endothelium-and/or muscle-derived relaxing factors.