5-Hydroxytryptamine (5-HT) is an important neurotransmitter and hormone/paracrine agent mediating various enteric functions. Its precise physiological and pathophysiological role remains unclear. This study investigated the effects of 5-HT on colonic function and the effects of the newly developed 5-HT3 and 5-HT4 receptor antagonist, FK1052, on colonic responses to 5-HT or stress stimulus in vivo. In conscious rats, both 5-HT and 5-methoxytryptamine significantly increased fecal pellet output and accelerated colonic transit. In contrast, the effect of 2-methyl-5-HT was slight. Although ondansetron and granisetron slightly reduced 5-HT (1 mg/kg s.c.) stimulated colonic transit, FK1052 [(+)-8,9-dihydro-10-methyl-7-[(5-methyl-4-imidazolyl)methyl]pyrido- [1,2-a]-indole-6(7H)-one hydrochloride], at 0.1 mg/kg p.o., inhibited completely the increases in the colonic transit. Furthermore, FK1052, ondansetron and granisetron significantly depressed the increase in fecal pellet output caused by wrap-restraint stress, with ED50 values of 0.21, 3.0 and 1.1 mg/kg p.o., respectively. Intraperitoneal administration of 5-HT and 5-methoxytryptamine, but not 2-methyl-5-HT, produced a dose-related increase in the incidence of diarrhea in fasted mice. 5-HT (0.32 mg/kg i.p.)-induced diarrhea was also inhibited by FK1052, ondansetron and granisetron, with ED50 values of 0.09, 2.3 and 0.88 mg/kg p.o., respectively. These findings suggest that 5-HT3 and 5-HT4 receptors may have an important role in colonic function and FK1052 may have therapeutic potential in the treatment of gastrointestinal dysfunction such as irritable bowel syndrome.