Zatosetron is being tested clinically as an antianxiety agent; it is a highly selective antagonist of the serotonin 5-HT3 receptor, with minimal agonist activity. The disposition of [14C]zatosetron was studied in five healthy men after a single oral dose (46.2 mg). Serum levels of radioactivity and parent drug peaked in 3-8 hr. About 15% more radioactivity was measured in red blood cells than in plasma. In serum, the parent compound represented about 85% of the radioactivity, zatosetron-N-oxide represented 10%, and N-desmethyl-zatosetron and 3-hydroxy-zatosetron each represented 2-3%. The t1/2 of zatosetron was 25-37 hr. About 75% of zatosetron added to human plasma became reversibly bound to protein. Concentrations of zatosetron in saliva were generally 10-50% higher than those in serum. About 80% of the administered radioactivity was eliminated in urine and 20% in feces; radioactivity was measurable in the excreta for 10-12 days after drug administration. The major route of metabolism of zatosetron was a stereoselective N-oxidation to form 8-alpha-methyl, 8-beta-oxo zatosetron (zatosetron N-oxide). In urine, approximately 45% of the radioactivity was unchanged zatosetron, 35% was zatosetron N-oxide, 10% was N-desmethyl-zatosetron, and 5% was 3-hydroxy-zatosetron. In feces, 30% of the radioactivity was unchanged zatosetron, and 70% was N-desmethyl-zatosetron. Overall, approximately 60% of the administered zatosetron was metabolized in humans. In a separate multiple-dose study, the disposition of zatosetron was found to be similar to that in the single-dose study.