Potent, selective 5-HT4 receptor antagonist (pKB = 9.43 in human colonic muscle, and Kd = 0.15 nM for binding to cloned human 5-HT4 receptors). Displays > 300-fold selectivity over 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C and 5-HT3 receptors.