This study characterized the functional effects of a novel gastroprokinetic agent, N-[2-(diisopropylamino)ethyl]-2-[(2-hydroxy-4,5-dimethoxybenzoyl)amino]-1, 3-thiazole-4-carboxyamide monohydrochloride trihydrate (Z338), on the muscarinic M1, M2, and M3 receptors expressed in Xenopus oocytes using the two-electrode voltage clamp method. Z-338 did not produce by itself any currents in oocytes expressing muscarinic M1, M3 receptors or muscarinic M2 receptors/G protein-gated inward rectifying K+ channels (Kir3.1 channels). In oocytes expressing muscarinic M1 receptors, Z-338 inhibited the acetylcholine-induced Ca2+ -activated Cl- current with an IC50 of 1.8 microM. In oocytes expressing muscarinic M2 receptors/Kir3.1 channels, Z-338 inhibited the acetylcholine-induced K+ currents with an IC50 of 10.1 microM, whereas in oocytes expressing muscarinic M3 receptors, Z-338 did not inhibit the acetylcholine-induced Ca2+ -activated Cl- current in a concentration-dependent manner. These results indicate that Z-338 is a potent antagonist not for muscarinic M3 receptor but for both muscarinic M1 and M2 receptors. Thus, Z-338 is a gastrokinetic agent with a unique profile.