The vasodilator effect of the ethanolic extract of Mansoa hirsuta leaves (EEF) was assayed in rat aortic rings. EEF produced a concentration-dependent vasodilatation (pIC(50)=5.1+/-0.2), which was absent in endothelium-denuded vessels. The vasodilator effect of EEF was similar to a standardized ethanolic extract of Hancornia speciosa Gomes (pIC(50)=5.1+/-0.1). The endothelium-dependent vasodilatation induced by EEF was abolished by L-NAME (100 microM), a nitric oxide (NO) synthase inhibitor, but not by indomethacin (10 microM; pIC(50)=4.9+/-0.2), a cyclooxygenase inhibitor. The concentration-response curve of EEF was not modified by the addition of superoxide dismutase (SOD; 300 U/ml). In addition, EEF (50 microg/ml) displaced the 3-morpholino-sidnonimine (SIN-1; p<0.05) concentration-effect curve to the left, as well as SOD (300 U/ml). These findings lead us to conclude that EEF induces a NO- and endothelium-dependent vasodilatation in rat aortic preparations, and that this effect is, at least in some extent, due to an increase in the NO bioavailability as consequence of its antioxidant activity. The HPLC-DAD profile recorded for EEF indicates the presence of four major peaks with close retention times, exhibiting similar UV spectra with wavelength maxima compatible with heterogeneous proanthocyanidins.