The aims of the present study were to investigate the vasoactive effects of luteolin and its mechanisms of action on the rat thoracic aorta. Luteolin (4.5-36 micromol/L) caused a concentration-dependent relaxation of endothelium-intact or endothelium-denuded aortic rings precontracted with phenylephrine (PE, 10(-6) mol/L) or a high level of K+ (6 x 10(-2) mol/L). Luteolin induced a shift of the PE concentration-response curve to the right and downward. L-NAME and propranolol did not influence the vascular effect of luteolin. However, 5-hydroxydecanoate, tetraethylammonium, BaCl2 and 4-aminopyridine significantly attenuated the vasorelaxant effect of luteolin. In Ca2+ -free medium, medium with graded concentrations of Ca2+, or K+ -free solution, luteolin reduced PE-induced contraction. It is concluded that luteolin induces endothelium-independent relaxation in rat thoracic aorta. The mechanism involves the inhibition of sarcolemmal Ca2+ channels, release from intracellular Ca2+ stores and activation of K+ channels.