The pharmacological profile of five quinoxaline derivatives, a new class of 5HT3 receptor antagonists, is reported in the present study. All of the new compounds antagonized the effect of the selective 5-HT3 receptor agonist 2-methyl-5-HT in the isolated longitudinal muscle-myenteric plexus preparation (LMMP) of guinea-pig ileum. One of them, VC-605) was approximately three orders of magnitude more potent than ondansetron. In binding studies to 5-HT3 receptors from rat cerebral cortex membranes only VC-605 showed an affinity comparable to ondansetron. In the isolated rat oesophageal tunica muscularis mucosae preparation the new compounds, like 2-methyl-5-HT, only produced relaxation of the contraction induced by carbachol at high concentrations. In vivo, the quinoxaline derivatives were weak antagonists of the bradycardia response to 5-HT in the anesthetized rat. The quinoxaline derivatives, in particular VC-501 and VC-603, prevented retches and vomiting induced by 2-methyl-5-HT and cis-platinum in the ferret. The new compounds also enhanced the gastric emptying of solids in rats. The results obtained are probably indicative of the suggested species- and tissue-dependent differences in 5-HT3 receptor subtypes. The high potency and selectivity of one of the new quinoxaline derivatives, VC-605, at 5-HT3 receptors of guinea-pig ileum is remarkable. VC-605 may be a useful tool for further characterizing this possible 5-HT3 receptor subtype.