Methyl brevifolincarboxylate (1) isolated from the leaves of Phyllanthus niruri L. showed a vasorelaxant effect on rat aortic rings. Compound 1 exhibited slow relaxation activity against norepinephrine (NE)-induced contractions of rat aorta with or without endothelium. The compound did not affect contractions induced by a high concentration (60 mM) of K⁺ , whereas it inhibited NE-induced vasocontraction in the presence of nicardipine. These results suggest that the inhibition of NE-induced vasocontraction by compound 1 is in part attributable to a decrease in [Ca²⁺]_i through receptor-operated Ca²⁺ channels.