Mebendazole (5-benzoyl-2-methoxycarbonylaminobenzimidazole) was highly effective against tetrathyridia of M. corti in the peritoneal cavity, liver and in the subcutis of mice when administered orally in the food (450 or 465 ppm) from days 7 or 40/50 post infection (p.i.) for 10 or 20 days in total doses of approximately 660-1660 mg/kg body wt. All tetrathyridia were killed when treatment began at day 7 p.i. and only some survived when medication was started on days 40/50 p.i. A treatment of 5 days was only partially effective. In experiments with E. multilocularis, laboratory animals were i.p. infected with metacestode tissue transplants. Beginning on days 4-7 p.i. or on day 40 p.i. the animals were treated for 5, 10 or 20/21 days with mebendazole medicated food. In mice a treatment for 10 or 20/21 days with total mebendazole doses of approximately 660-2180 mg/kg body wt caused an average reduction of metacestode weight of 61-89%. An average reduction between 58-84% was achieved in Meriones after a treatment for 20/21 days with total doses of approximately 650/830 mg/kg body wt. Histological examinations and transplantation experiments revealed that in most of the treated animals metacestode tissue survived and retained its capability for budding. Indications for a partial destruction of the parasite tissue were found in some of the treated animals. The oral mebendazole treatment was well tolerated. After i.p. injection of 3 .times. 150 mg/kg body wt mebendazole, all of 36 mice or Meriones died.