The objectives of the present study were, first, to establish a methodology for evaluation of the permeability in vitro of hydatid cysts to different drugs and, second, to compare the permeability to albendazole sulfoxide of cysts from untreated animals, cysts from animals treated with 50 mg/kg netobimin for 5 days, and cysts from animals treated with 50 mg/kg netobimin plus 1.1 mg/kg fenbendazole for 5 days. The drug flow follows the Fick law, i.e., the uptake occurs by simple diffusion. We calculated the permeability constant of the cyst membrane by taking into account the disappearance velocity constant, the cyst area, and the incubation solution volume. The permeability value obtained for albendazole sulfoxide was 8.06+/-2.30 x 10(-6) cm s(-1) in cysts from untreated animals, 5.56+/-2.53 x l0(-6) cm s(-1) in cysts from animals treated with netobimin, and 7.05+/-3.04 x 10(-6) cm s(-1) in cysts from animals treated with netobimin +/- fenbendazole. These permeability values show significant differences (P < 0.05).