The long-lasting antihypertensive effect of a neutral extract of Bidens pilosa has been suggested to be due to vasodilation. The present work was undertaken to assess this hypothesis. The vasorelaxant effect of a neutral extract (NBp) of the leaves of B. pilosa was evaluated in vitro on isolated rat aorta contracted with KCl or norepinephrine. NBp induced a concentration-dependent vasorelaxation of the rat aorta precontracted with KCl (60 mM) by 25%–105% at the respective concentrations of 0.25–1.5 mg[sol ]mL. The maximal concentration of 1.5 mg[sol ]mL provoked 88% relaxation of norepinephrine-induced contractions. There were no significant differences between the effects of the extract on the aorta strips with or without endothelium. In the presence of indomethacin or pyrilamine maleate, the relaxant response induced by the plant extract was significantly inhibited at the lower concentrations. The plant extract was able to reduce the aorta resting tone, inhibit the KCl-induced contractions (90% at 1.5 mg[sol ]mL) and the CaCl2-induced contractions by 95% at a concentration of 0.75 mg[sol ]mL. These results demonstrate the vasodilating properties of the neutral extract of Bidens pilosa and indicate that it may act as a calcium antagonist. Copyright © 2005 John Wiley & Sons, Ltd.