Albendazole pharmacokinetic parameters were determined in lambs after iv, oral, and intraruminal single administrations. The parent drug and two metabolites, albendazole sulfoxide and albendazole sulfone, were simultaneously determined in whole blood, plasma, and urine using an HPLC method. The parent drug was only recovered in plasma when injected intravenously. For other routes, only the two metabolites were detectable; they were present in red blood cells and plasma at equal concentrations. The pharmacokinetic parameters were determined by using compartmental models which simultaneously described the two oxidative steps and the urinary excretion of the sulfoxide derivative. Dose-dependent pharmacokinetics was studied in the dose range 0.95-3.8 mg/kg. The results showed that clearance remained constant within the tested dose range since the area under the curve normalized to the dose was similar in the cases of sulfoxide and sulfone metabolites, whatever the route of administration. The drug appeared to be extensively metabolized in the body regardless of the route of administration. Sulfoxidation probably took place in liver, but other tissues seemed to be responsible for the formation of the sulfoxide which has been described as the major anthelmintic derivative of albendazole.