AbstractPharmacokinetics of albendazole sulphoxide (ABZ‐SO) in three different single oral doses of albendazole (ABZ) (400, 800 and 1200 mg) was studied in 10 healthy human volunteers in a double blind three‐way crossover design. The serum levels of albendazole main metabolite, albendazole sulphoxide (ABZ‐SO), were analysed by a modified high‐pressure liquid chromatography method. (ABZ is not detectable in biological fluids itself.)For ABZ‐SO, there was no significant difference in the biological half life, normalized serum peak concentration (Cmax−ABZ−SO/DoseABZ), time to reach peak concentration (Tmax) and mean residence time (MRT), whereas apparent clearance (Clp/F), apparent distribution volume (Vd/F), normalized area under the serum concentration‐time curve (AUCABZ−SO/DoseABZ) and normalized area under the first moment curve (AUMCABZ−SO/DoseABZ) of albendazole main metabolite (ABZ‐SO) were statistically different at different doses of the parent drug, resulting in substantially lower serum concentration and thereafter AUCABZ−SO/DoseABZ and AUMCABZ−SO/DoseABZ in higher doses. These observations indicate dose dependent pharmacokinetics of albendazole (observed for ABZ‐SO), which were explained on the basis of a change in fraction of dose absorbed (F) as a result of slow and incomplete dissolution of the main drug in the GI tract. Copyright © 2002 John Wiley & Sons, Ltd.