Involvement of ileal and circulating serotonin (5-HT) levels in cisplatin-induced emesis was examined using a microdialysis technique and an extraction method in dogs. The 5-HT levels in the ileal dialysate were increased to 232-294% of the basal level from 100 to 180 min after cisplatin administration (3 mg/kg, i.v.) and had returned to the basal level 280 min after dosing. The 5-HT levels in the blood dialysate were increased to 424-2165% from 140 to 180 min after dosing. The concentrations of 5-HT determined by HPLC following extraction were increased to 271% in the ileal mucosa and to 478% in plasma 3 hr after dosing. In immunohistochemistry, the number of 5-HT-immunoreactive cells was increased to 166% in the ileal mucosa following cisplatin treatment. These results strongly suggest that increases in the release and synthesis of 5-HT in the gut, probably in the enterochromaffin cells, are intimately involved in cisplatin-induced emesis.