The mode of antagonism of 5-hydroxytryptamine-induced positive inotropic effects by the highly selective 5-HT4 receptor antagonist SB 207710 (1-butyl-4-piperidinyl) methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate) was investigated on isolated preparations of human right atrial appendage. SB 207710 caused concentration-dependent (0.1-10 nmol/l) surmountable antagonism of the effects of 5-hydroxytryptamine with a pKB (mol/l) of 10.1. Due to its high selectivity and affinity, SB 207710 could be a powerful tool for the comparison of human atrial 5-HT4 receptors with 5-HT4 receptors of other organs of man and other species.