Abstract. The effect of osthole, isolated from Angelica pubescens, on the contraction of guinea-pig trachea was studied. Osthole (25100mol/l), theophylline (101000mol/l) and higher concentrations of nifedipine (0.1100mol/l) suppressed the contraction response curves of tracheal smooth muscle caused by carbachol, prostaglandin F2 (PGF2), U46619 (thromboxane A2 analogue) and leukotriene C4 (LTC4) in a concentration-dependent manner. The contraction caused by high K+ (120mmol/l) and cumulative concentrations of CaCl2 (0.033mmol/l) was also inhibited concentration-dependently by osthole (25100mol/l), theophylline (101000mol/l) and lower concentrations of nifedipine (0.010.1mol/l). The relaxant actions of osthole were not affected by propranolol (1mol/l), glibenclamide (10mol/l) or removal of tracheal epithelium. Osthole (100mol/l) was still effective in causing tracheal relaxation in the presence of nifedipine (1mol/l). In Ca2+-free- and EGTA (0.2mmol/l)-containing medium, the relaxing effect of osthole was more potent than in normal Krebs solution. Osthole (25 and 50mol/l) caused 2.9 and 6.5, or 3.0 and 5.6 fold, respectively, increase in potency of forskolin or sodium nitroprusside in causing tracheal relaxation but did not affect that by cromakalim. Osthole (50mol/l) enhanced the increase in tissue cAMP and cGMP levels induced by forskolin and sodium nitroprusside, respectively, and in higher concentrations (100 and 250mol/l), itself increased markedly tissue cAMP and cGMP contents. Osthole (10250mol/l) inhibited the activity of cAMP and cGMP phosphodiesterases in a concentration-dependent manner. It is concluded that osthole exerts a non-specific relaxant effect on the trachealis by inhibiting the cAMP and cGMP phosphodiesterases.