Microdesmis keayana (Pandaceae) is an African tropical plant whose roots are used in traditional medicine for erection impairment but the compounds responsible for its action are unknown. Two major alkaloids recently isolated from the roots of M. keayana, keayanidine B and keayanine, were tested for vasorelaxing properties using isolated rat aortic rings precontracted by phenylephrine to confirm its traditional use. Influence of the alkaloids on the endothelial production of endothelial nitric oxide synthase (eNOS) was measured by quantitative polymerase chain reaction (QPCR) analysis. Scavenging activities were assessed versus 1,1-diphenyl-2-picrylhydrazyle (DPPH) and reactive oxygen species (ROS) such as superoxide anion (O2 •-) and hydrogen peroxide (H2O2) in cell-free and cellular systems. The results showed that keayanidine B and keayanine had significant vasorelaxing properties. This effect could be due to their strong antioxidant activity versus O2 •- and H2O2 and to their stimulation of eNOS mRNA expression. Therefore these alkaloids could indirectly stimulate NO production in the vascular bed and would explain the traditional use of M. keayana in erectile dysfunction. Copyright © 2008 John Wiley & Sons, Ltd.