The disposition of 14C-labelled fenflumizole was studied in five healthy subjects who received 0.1 mg/kg of fenflumizole as single intravenous doses. Radioactivity was measured in whole blood, plasma, urine and feces from prior to until 168 hours after dose administration. Chemical determinations of fenflumizole and its two demethylated derivatives in plasma were made by HPLC followed by fluorescence detection. Concentrations of fenflumizole calculated from radioactivity in plasma were consistently higher than from chemical determinations, suggesting the presence of metabolites. Radioactivity in blood cells decreased more slowly than in plasma. Plasma concentrations versus time could be described by a four-compartment model, with a terminal half-life of 119 hours (median). About 50% of the dose was excreted with the faeces, and about 4% was recovered in the urine. Unchanged fenflumizole in the urine accounted for 0.0001% of the total dose. Renal clearance of radioactive material decreased at low concentrations of radioactive compound in plasma, which may indicate changes in unbound fraction of drug and metabolites, or a saturable tubular reabsorption process. The slow elimination of fenflumizole is consistent with slow release from tissues and/or enterohepatic recycling.