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The effect of an aqueous extract of Morinda lucida (ML) on vascular tone was tested using rat aortic rings precontracted with noradrenaline (10⁻⁷ mol/l). Relaxation responses were determined with endothelium-intact, L-NAME-treated, methylene blue-treated and endothelium-denuded tissues. In the concentration range of 0.25–9.0 mg/ml, ML elicited vasorelaxation in noradrenaline-precontracted rings. This relaxation response was partially attenuated by removal of the endothelium, and completely inhibited by pretreatment of rings with L-NAME and methylene blue. Thus, ML-induced relaxation of vascular smooth muscle occurs via endothelium-dependent and -independent mechanisms, the former of which involves the nitric oxide-cGMP pathway.