In our previous study, we showed that methanolic extracts from Combretum laxiflorum leaves (ClLv), Combretum racemosum leaves (CrLv) and bark root (CrBR) and Hymenocardia acida bark trunk (HaBTr) and bark root (HaBR) have an endothelium-dependent vasorelaxant effect on isolated rat thoracic aorta. These plants are used as antihypertensive agents in RDCongo. The present study was conducted to investigate the mechanism involved in this response and to assess the activity of fractions obtained from the more active extracts (ClLv and HaBR). To obtain the fractions, the methanolic extract was suspended in ethanol and mixed with polyamide powder (5g). A glass column was filled with this mixture and eluted in three steps: fraction 1: ethanol; fraction 2: ethanol-acetone-water (80:16:4); fraction 3: acetone-water (7:3). To explore the involvement of vasodilator prostanoids and soluble guanylyl cyclase/ cyclic guanosine 3,5-monophosphate (GC/cGMP) in the vasorelaxant activity of our congolese plant extracts and fractions, rings of rat aorta were pretreated with 10 mM indomethacin, a cyclooxygenase inhibitor or 10 mM ODQ, a guanylyl cyclase inhibitor. The plant extract or the fraction was applied cumulatively (0.1–50mg/ml) after contraction with 1 mM phenylephrine. The active fraction was also tested on rubbed aorta rings and in the presence of 100 mM L-NAME, a NO synthase inhibitor. The vasorelaxant activity of all the tested extracts was inhibited by ODQ. Indomethacin only inhibited the activity of CrLv and CrBR extracts. The responses induced by HaBR and ClLv was obtained in fraction 3 and reached respectively 100.3 ± 3.5% and 96 ± 2.8% at 3mg/ml. This effect was significantly attenuated by endothelium removal and after pretreatment with L-NAME and ODQ but not with indomethacin. Methanolic extracts from ClLv, HaBTr and HaBR induce an endothelium-dependent vasorelaxation through the NO-cGMP pathway while CrLv and CrBR extracts also act via a vasodilator prostanoid. Fractions 3 containing procyanidins seem to be responsible for the activity of ClLv and HaBR extracts.