The purpose of this study was to examine the in vivo effects of Ekebergia capensis leaf ethanolic extract (EKE) on the blood pressure of anaesthetised normotensive male Wistar rats and conscious weanling Dahl salt-sensitive (DSS) rats, which develop hypertension as they age. To investigate possible mechanism(s) of the extract's hypotensive effects, the contractile or relaxant responses to EKE in the absence or presence of reference drugs were evaluated in Wistar rat isolated aortic rings precontracted with methoxamine hydrochloride (ME, 10 microM). Acute intravenous administration of EKE elicited hypotensive responses in anaesthetised animals, while sub-chronic treatment with the extract averted the development of high blood pressure in weanling DSS rats. Isometric recordings of methoxamine hydrochloride (ME) pre-contracted, isolated, endothelium-intact and -denuded aortic rings revealed concentration-dependent relaxation responses to EKE (1-160 mg/ml). The potency was significantly less in the endothelium- denuded rings. Inhibitors of endothelium-derived relaxing factor (EDRF), L-NAME, methylene blue and indomethacin significantly reduced EKE-evoked vasorelaxations in endothelium-intact aortic rings. These results indicate that the vasorelaxant effect of EKE was in part mediated via EDRF-dependent or -independent pathways. These observations suggest that the hypotensive effect of EKE was in part mediated via modulation of total peripheral resistance of the vascular smooth muscles.