The leaves and flowers of Verbascum species are used to treat respiratory disorders, haemorrhoids, rheumatic pain, and wounds as well as for the treatment of eczema and other types of inflammatory skin conditions in traditional Turkish medicine. We examined the effect of the methanolic extract of the aerial parts of Verbascum latisepalum Hub.-Mor. on the endothelium-dependent relaxation response in rat aortic rings which is mediated by nitric oxide (NO). Six fractions, A F, were obtained from the methanolic extract through bioassay-guided fractionation procedures. The phenylethanoid glycoside verbascoside was isolated from fraction D and its structure elucidated by spectral techniques. The inhibitory effects of the extract, its fractions, and verbascoside on the acetylcholine-induced relaxation response in phenylephrine-precontracted aorta was examined in the absence and presence of L-arginine, a precursor in the synthesis of NO. The observation that the effects of the methanolic extract, of fraction D, and of verbascoside were reversed by L-arginine, indicates that verbascoside has an inhibitory effect on the synthesis of NO. This effect should be taken into consideration in view of the wide range of uses of Verbascum species in Turkish folk medicine.