SR 57227A (4-amino-(6-chloro-2-pyridyl)-1 piperidine hydrochloride) is a novel compound with high affinity and selectivity for the 5-HT3 receptor. The compound had affinities (IC50) varying between 2.8 and 250 nM for 5-HT3 receptor binding sites in rat cortical membranes and on whole NG 108-15 cells or their membranes in vitro, assayed under various conditions with [3H]S-zacopride or [3H]granisetron as radioligand. Like reference 5-HT3 receptor agonists, SR 57227A stimulated the uptake of [14C]guanidinium into NG 108-15 cells in the presence of substance P (EC50 = 208 +/- 16 nM) and contracted the isolated guinea-pig ileum (EC50 = 11.2 +/- 1.1 microM), effects that were antagonised by the 5-HT3 receptor antagonist tropisetron. The agonist effect of SR 57227A was also observed in vivo, as the compound elicited the Bezold-Jarisch reflex in anesthetised rats (ED50 = 8.3 micrograms/kg i.v.), an effect that was blocked by tropisetron and R,S-zacopride, but not by methysergide. When injected unilaterally into the mouse striatum, SR 57227A, like 2-methyl-5-HT, elicited contralateral turning behaviour which was antagonised by ondansetron. Furthermore, microiontophoretic application of SR 57227A markedly inhibited the firing rate of rat cortical neurones, an effect antagonised by tropisetron. Finally, in contrast to reference 5-HT3 agonists, SR 57227A bound to 5-HT3 receptors on mouse cortical membranes after systemic administration (ED50 = 0.39 mg/kg i.p. and 0.85 mg/kg p.o.). These results suggest that SR 57227A is a potent agonist at peripheral and central 5-HT3 receptors, both in vitro and in vivo. In view of the dearth of 5-HT3 receptor agonists which are capable of crossing the blood-brain barrier, SR 57227A may be useful in the characterisation of the neuropharmacological effects produced by the stimulation of these receptors.