Moringa oleifera seeds are widely used to manage hypertension in traditional medicine in the northern part of Madagascar. In this work we studied the vasodilatory activity of a molecule isolated from the seeds for the first time 4ʹ-((3,4,5-trihydroxy-6-(1-hydroxyethyl)-tetrahydro-2H-pyran-2-yl)oxy)benzylamino-3ʹ-(2-hydroxyethane) that we coded N-428. It was isolated from ethyl acetate extract. It relaxes rat thoracic aorta contracted with norepinephrine at 10 µM with EC50 equal to 8.92.10-5 M. Further research revealed that it can dilate endothelium denuded aorta and promotes vasodilation, it provokes a right shift of the concentration-effect curve of norepinephrine without changing the maximal effect, indicating its activity via α1 receptors. Compared to prazosin, a competitive antagonist of norepinephrine its pA2 is 5.78 M, versus 8.249 for prazosin, and Kd of 1.25.10-6 M, versus 5.63.10-9 M for prazosin. These results indicate that the isolate exhibits good vasodilatory activity, acting as α1 competitive antagonist of norepinephrine and has the potential to be utilized as a lead vasodilator compound.