This study was undertaken to investigate the putative mechanism(s) underlying the antispasmodic effect of 7-epiclusianone, a naturally occurring compound isolated from the plant Garcinia brasiliensis. Guinea pig tracheal rings were mounted in tissue baths filled with Krebs' solution, and the contractile response to distinct stimuli was measured in the presence or absence of 7-epiclusianone. We also tested the effect of 7-epiclusianone on methacholine-evoked airways obstruction in BALB/c mice using barometric plethysmography. 7-Epiclusianone (10 M) inhibited epithelium-intact tracheal ring contraction induced by allergen, histamine, 5-hydroxytryptamine, or carbachol challenge. The relaxation effect was abrogated by epithelium removal, the presence of nitric-oxide synthase inhibitor N -nitro-L-arginine methyl ester (L-NAME) (100 M), or soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (10 M). 7-Epiclusianone (1-100 M) induced a dosedependent increase in the intracellular cGMP levels of cultured tracheal rings. The relaxation effect of 7-epiclusianone was also inhibited by K ϩ channel blockers tetraethylammonium (10 M), glibenclamide (1 M), or apamin (1 M), but not by 9-(tetrahydro-2Ј-furyl)adenine (SQ22,536) (100 M), an adenylate cyclase inhibitor. In epithelium-intact tracheal rings, 7-epiclusianone also inhibited Ca 2ϩ -induced contractions in K ϩ (60 mM)-depolarized preparations, but it seemed ineffective in assays in which epithelium-denuded tracheal ring preparations were used. Oral administration of 7-epiclusinone (25-100 mg/kg) dose-dependently inhibited airway obstruction triggered by aerosolized methacholine (6 -25 mg/ml), in a mechanism sensitive to L-NAME (20 mg/kg). In conclusion, the relaxation effect of 7-epiclusinone seems to be mediated by epithelium-, nitric oxide-, and cGMP-dependent mechanisms. Furthermore, oral administration of 7-epiclusianone reduces episodes of bronchial obstruction, warranting further research on this compound regarding a putative application in asthma therapy.Ginkgo biloba, Artocarpus heterophyllus, Allium sativum, Glycyrrhiza glabra, and Panax ginseng ginseng are examples of plant species that have been used in the so-called complementary and alternative medicine as herbal formulations,