Functional 5-HT4 receptors have been reported to be present in numerous isolated tissue preparations including the rat oesophagus, guinea-pig ileum, and human colon. The pharmacological properties of the novel, potent and selective 5-HT4 receptor antagonists SB203186 (1-piperidinyl)ethyl 1H-indole 3-carboxylate), SB205008 (1-butyl-1-methyl-4-piperidinylmethyl)-8-amino-7-chloro-1,4-ben zodioxan-5- carboxylate iodide), and SB207710 (1-butyl-4-piperidinylmethyl)-8-amino-7-iodo-1,4 benzo-dioxan-5-carboxylate) were studied in these tissues. The nature of antagonism of the 5-HT-induced effects was investigated on the above isolated tissue preparations. 5-HT produced its effect with the following EC50 values: 400 +/- 0.4 nM (rat oesophagus, n = 20), 154 +/- 14 nM (guinea-pig ileum, n = 9) and 144 +/- 0.1 nM (human colon, n = 9). SB207710 (0.03-1 nM), SB205008 (1.0-10 nM), and SB203186 (10-100 nM) antagonised the 5-HT4 receptor-mediated relaxations of the carbachol-contracted rat isolated oesophagus against 5-HT with pKB values of 10.9 +/- 0.1, 9.5 +/- 0.1, and 9.0 +/- 0.1 respectively without effecting the maximum response. On the guinea-pig ileum peristaltic reflex preparation, SB207710 (0.01-1 nM) did not modify the reflex but it behaved as an antagonist of the 5-HT-induced facilitation with a pA2 value of 9.9 +/- 0.2.(ABSTRACT TRUNCATED AT 250 WORDS)