1. Baicalin was isolated and purified from the dry roots of Scutellaria baicalensis Georgi (Huangqin; a traditionally used Chinese medicinal herb) and its effect on the contractility of rat isolated mesenteric arteries was investigated and the role of the endothelium was examined. 2. The concentration-dependent contractile response to U46619 was enhanced by 10(-5) mol/L baicalin in endothelium-intact rings, but this effect was abolished in the presence of 10(-4) mol/L N(G)-nitro-L-arginine or in endothelium-denuded rings. 3. Pretreatment of endothelium-intact rings with baicalin (3 x 10(-5) mol/L) markedly attenuated the relaxant response to A23187, thapsigargin and acetylcholine. 4. The present results indicate an important role for endothelial nitric oxide (NO) in the vascular response to baicalin. Baicalin appears to inhibit NO production and release in the endothelium and this mechanism is likely to be responsible for the enhancement of the U46619-induced contraction and for inhibition of endothelial NO-mediated relaxation by baicalin in rat mesenteric artery.