The effects of mebendazole (Meb), albendazole (Alb) and albendazole sulfoxide (AlbSO) on metacestodes of Echinococcus granulosus have been studied. The results show that Meb and Alb exhibit less effect on protoscoleces in vitro, but in vivo the protoscolicidal effect of Meb is higher than that of Alb. Both Meb and Alb are efficacious in the treatment of mice infected with secondary cysts of E. granulosus. In terms of the minimal effective dose required, the occurrence of collapsed and shrunk cysts after treatment, the drug-induced damage on the germinal layer and the relationship between the drug content in the cyst wall and the damage of germinal layer, the effect of Meb on metacestodes of E. granulosus is higher than that of Alb. The results also indicated that lower drug absorption rate and thickening of the adventitia during longer disease course are the two major factors affecting the efficacy of Meb and Alb, hence suggesting that increase of Meb absorption may be expected to raise the therapeutic effect of the drug. Finally, Alb is not only metabolized to AlbSo and AlbSP in vivo, but also metabolized by the cysts exposed to Alb in vitro. Experimental chemotherapy showed that AlbSO is the major effective metabolite of Alb, and its curative dose is only one half of the parent compound.