The n-butyl quaternary salt of the 5-HT4 receptor agonist and 5-HT3 receptor antagonist, renzapride, is reported to be a potent and selective 5-HT4 receptor agonist. Serotonin - 5-HT4 receptors have been identified in mouse embryo coliculli neurones1, guinea pig hippocampus2, the gastrointestinal tract3,4 and in both piglets and human atrium6. The present communication describes the results we have obtained by quaternising the potent 5-HT4 receptor agonist, renzapride (BRL 24924)7,8. Renzapride is also a potent 5-HT3 receptor antagonist9, and for selectivity for the 5-HT4 receptor, we required a relative reduction in this activity. It has been reported that methyl quaternary salts of 5-HT3 receptor antagonists, such as ICS 205-930, retain or even increase 5-HT3 receptor antagonist potency10. However, for free bases, N-substituents larger than methyl dramatically reduce this activity. We therefore quaternised renzapride by reaction with n-butyl bromide in EtOH at reflux to give SB 205149 to investigate its pharmacological properties. SB 205149 was assessed for its agonist potency and efficacy at the 5-HT4 receptor located in the rat oesophageal tunica muscularis mucosae3 and its 5-HT3 receptor affinity by its ability to displace [3H] granisetron from rat cortex (see Table). The previously published results for metoclopramide, renzapride7, BIMU 1 and BIMU 812,13 are included for comparison. SB 205149 was found to be a potent and highly effective 5-HT4 receptor agonist with a potency approximately 6 fold greater than renzapride, equivalent to 5-HT and almost 100 fold more potent than metoclopramide. However, as expected, SB 205149 was found to be only a weak 5-HT3 receptor antagonist with a pKi of 6.9, similar to that of metoclopramide. In contrast both BIMU 1 and BIMU 8, although highly potent 5-HT4 receptor agonists, are also highly potent 5-HT3 receptor antagonists. In conclusion, n-butyl quaternisation of renzapride both reduced 5-HT3 receptor affinity and increased 5-HT4 receptor agonist potency to give a potent and selective 5-HT4 receptor agonist. In addition, being a quaternary derivative, the compound is very polar and, if radiolabelled, could provide a suitable ligand for radioligand binding studies.