Ondansetron (GR38032F) is a selective antagonist of 5-hydroxytryptamine (5-HT) type 3 receptors. This randomized, double-blind, cross-over study was undertaken to evaluate and compare the effect of ondansetron with placebo on gastrointestinal transit in 10 healthy male volunteers. There were no significant differences between the effects of placebo and ondansetron on gastric emptying or mouth-to-caecum transit time. However, significant differences in mean whole-gut transit time were observed, that is 54.8 h with ondansetron and 32.1 h with placebo. Therefore, 5-HT3 receptors may be involved in the regulation of colonic transit and ondansetron may prove useful as an anti-diarrhoeal agent.