This study reports the effect of Spilanthes acmella Murr. extracts on phenylephrine-induced contraction of rat thoracic aorta as well as their antioxidant activity. Results show that the extracts exert maximal vasorelaxations in a dose-dependent manner, but their effects are less than acetylcholine-induced nitric oxide ( NO) vasorelaxation. Significant reduction of vasorelaxations is observed in both N-G-nitro-L-arginine methyl ester ( L-NAME) and indomethacin ( INDO). In the presence of L-NAME plus INDO, synergistic effects are observed, leading to loss of vasorelaxation of both acetylcholine and the extracts. Similarly, the vasorelaxations of the extracts are completely abolished upon the removal of endothelial cells. This demonstrates that the extracts exhibit vasorelaxation via partially endothelium-induced NO and prostacyclin in a dose-dependent manner. Significantly, the ethyl acetate extract exerts immediate vasorelaxation ( ED50 76.1 ng/mL) and is the most potent antioxidant ( DPPH assay). The chloroform extract shows the highest vasorelaxation and antioxidation ( SOD assay). These reveal a potential source of vasodilators and antioxidants.