Recently we reported that GS 386, 1-(4 -methoxybenzyl)-6,7-dimethoxy-3,4-dihydroisoquinoline, inhibited amplitude of the Ca²⁺ current by reducing the probability of Ca²⁺ channel opening without changing channel kinetics in isolated rabbit atrial myocyte. In the present study, further investigation of the mechanism of action of GS 386 was performed using isolated rat trachealis. GS 386 concentration-dependently relaxed rat trachealis contracted by carbachol (0.3μM) and high K⁺(65.4 mM) with IC₅₀ 5.24 and 5.67 μM, respectively. Verapamil inhibited more effectively the high K⁺-contracted tissues than those with carbachol in the rat trachealis muscle. In Ca²⁺-free media, Ca²⁺-induced contraction was inhibited by GS 386. Furthermore, high concentration of GS 386 (100μM) but not verapamil, attenuated a phasic contraction induced not only by carbachol but also caffeine, indicating that GS386 can enter into the cytoplasm where it may exert secondary actions on internal sites of the muscle, such as sarcoplasmic reticulum. Moreover, GS 386 showed verapamil-resistant component of relaxation and increased cAMP levels in rat trachal smooth muscle. These results suggest that the mechanism of action of GS 386 attributes to not only Ca²⁺ antagonistic action but also weak phosphodiesterase inhibitory action.