The excretion and biotransformation of cisapride, a novel gastrokinetic drug, were studied after single (10, 40, and 160 mg/kg) and repeated (10 mg/kg/day) po administration to rats, using three different radiolabels. In fasted rats, cisapride was absorbed almost completely, except for the 160 mg/kg dose. Cisapride was metabolized extensively to at least 30 metabolites. The excretion of the metabolites amounted to more than 80% of the dose at 24 hr and was almost complete at 96 hr after dosing. In bile duct-cannulated rats, 60% was excreted in the bile within 24 hr, 45% of which underwent enterohepatic circulation. The main urinary metabolites, 4-fluorophenyl sulfate and norcisapride, primarily resulted from the N-dealkylation at the piperidine. Another major metabolic pathway was aromatic hydroxylation, occurring on either the 4-fluorophenoxy or the benzamide rings. The resulting phenolic metabolites were eliminated as conjugates in the bile; a large portion of them were subjected to a rapid enterohepatic circulation before their final excretion in the feces. Minor metabolic pathways included piperidine oxidation, O-dealkylation, O-demethylation of the methoxy substituent at the benzamide, and amine glucuronidation. Only minor quantitative dose- and sex-dependent differences could be observed for the mass balance of the metabolites. Upon repeated po dosing, steady state excretion rates were already attained after two to three doses, and excretion and metabolite patterns were very similar to those after single dose administration.