Based on screening for vasoactive traditional Chinese medicinal herbs, the present study was performed to investigate the vasoactive effects of an ethyl acetate extract from leaves of Morus alba (L.) (ELM) on rat thoracic aorta and the mechanisms underlying these effects. Isolated rat thoracic rings were mounted in an organ bath system and the effects of ELM on their responses were evaluated. ELM (0.125-32.000g/l) induced a concentration-dependent relaxation (P<0.01 vs. control) both in endothelium-intact and -denuded aortas precontracted by high K(+) (6 x 10(-2)M) or 10(-6)M phenylephrine (PE). In endothelium-denuded aortas, ELM at the EC(50) concentration reduced Ca(2+)-induced contraction (P<0.01 vs. control) after PE or KCl had generated a stable contraction in Ca(2+)-free solution. And after incubation with verapamil, ELM induced contraction in endothelium-denuded aortas precontracted by PE (P<0.01 vs. control); this was abolished by ruthenium red (P<0.01 vs. ELM-treated endothelium-denuded group; P>0.05 vs. control), but not by heparin (P>0.01 vs. ELM-treated endothelium-denuded group; P<0.01 vs. control). The results showed that ELM had dual vasoactive effects, and the relaxation was greater than the contraction. The relaxation was mediated by inhibition of voltage- and receptor-dependent Ca(2+) channels in vascular smooth muscle cells, while the contraction occurred via activation of ryanodine receptors in the sarcoplasmic reticulum.