Novel modulators of 5-HT4 receptors have been developed which have a selectivity for peripheral receptors rather than those of the central nervous systems. Theses include novel derivatives of known modulators as well as entirely novel entities. Surprisingly, the derivatised compounds of the known modulators maintain a high binding affinity to 5-HT4 receptors, despite the presence of an acidic moiety at the end of an optional chain. The entirely novel entities also exhibit good binding affinity to 5-HT4 receptors. All of the compounds of the invention have a common motif which includes a basic nitrogen moiety and an acidic moiety. The compounds of the invention, due at least in part to their high ionisation potential at physiological pH, have the unique properties of selectively for peripheral 5HT4 receptors over those of the CNS, good binding affinity, and selectively of 5HT4 receptors over other serotonin receptors.