Combretum laxiflorum Welw (Combretaceae), Combretum racemosum P. Beauv (Combretaceae) and Hymenocardia acida Tul (Euphorbiaceae) are traditionally used in RDCongo for the treatment of many diseases including cardiovascular disorders. To our best knowledge, there are no data on vascular relaxant effect of these medicinal plants. We assessed the vasorelaxant activity of extracts obtained from these plants in isolated rat thoracic aorta rings. Methanol extract of Combretum laxiflorum leaves (ClLv), Combretum racemosum leaves (CrLv) and bark root (CrBR) and Hymenocardia acida bark trunk (HaBTr) and bark root (HaBR) were prepared. Rings of aorta from male rats (Wistar), with or without intact endothelium, were mounted in tissue baths. After contraction with phenylephrine (Phe, 1mM) the plant extract was applied cumulatively (0.1-100mg/ml). In some experiments, prior to the addition of the extract, rings were pre-treated for 30 minutes with 100mM N^G-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, followed by the addition of 1mM Phe to obtain a similar contractile activity as in vessel rings not treated with L-NAME. All the tested extracts exhibited concentration-dependent vasorelaxations of Phe-induced contractions of intact aortic rings. HaBTr (5mg/ml), HaBR (10 mg/ml) and ClLv (10 mg/ml) methanol extracts potently relaxed aortic rings by respectively 100.4 ± 2.0%, 93.9 ± 1.5% and 85.1 ± 3.0%. The maximum vasorelaxant responses of CrBR and CrLv methanol extracts were respectively 52.7 ± 4.9% and 48.7 ± 9.5% at a cumulative concentration of 100 mg/ml. The activity disappeared or was significantly attenuated by removal of functional endothelium or pre-treatment of the aortic tissues with L-NAME. After removal of the endothelium, the maximum relaxation was less than or equal to 10% for all the extracts except for CrBR (p>0.05). Similar inhibitions were obtained with L-NAME incubation. These results support a significant role of endothelium/NO in these medicinal plants extracts-induced relaxations of rat aorta. The present study showed that the vasorelaxant effect of methanolic extracts from ClLv, CrLv, HaBTr and HaBR seems mainly mediated by the endothelial NO signaling in aortic tissues.