BACKGROUND The antineoplastic drug cisplatin has been widely used for the treatment of cancer in humans but its use has been limited by vomiting and diarrhoea. Cisplatin releases 5-hydroxytryptamine into the gut which is thought to be the major mediator of cisplatin induced vomiting. OBJECTIVE To determine whether cisplatin affects fluid and electrolyte transport in rat jejunum and whether this change can be modulated by the 5-hydroxytryptamine3 receptor antagonist, ondansetron. METHODS Jejunal perfusion in rats in vivo was performed one hour after intraperitoneal cisplatin (5 and 10 mg/kg) administration. The effect of pretreatment with subcutaneous ondansetron 300 microg/kg was investigated. RESULTS Median net fluid absorption after cisplatin 10 mg/kg (67 microl/min/g dry intestinal weight (interquartile range 46 to 100); n = 15) was reduced compared with controls (120 (107 to 151) microl/min/g; n = 13; p<0.001). Ondansetron reversed the impairment of jejunal fluid absorption produced by cisplatin to normal (161 (130 to 176) microl/min/g; n = 11; p<0.001). Electrolyte movement paralleled fluid movement. Jejunal histological examination of sections from cisplatin treated animals showed villus damage, which was not prevented by pretreatment with ondansetron. CONCLUSIONS These findings suggest that diarrhoea during cisplatin therapy may be due to altered fluid transport in the small bowel. The reversal of fluid transport to normal in the presence of a 5-hydroxytryptamine3 receptor antagonist suggests that 5-hydroxytryptamine is a local mediator in the small intestine.