Mebendazole (Meb), albendazole (Alb) or albendazole sulfoxide (AlbSO) were taken rapidly in vitro by secondary cysts of Echinococcus granulosus removed out from mice infected with protoscoleces 8-9 months previously. The amounts of the drugs taken by the cysts were apparently increased followed by exposure of the cysts to the drugs at 10 micrograms.ml-1. The Alb penetrated into the cysts was distributed mainly in cyst wall, whereas the content of Meb in cyst wall was twice as much as that in cyst fluid. The distributions of AlbSO in cyst wall and cyst fluid were similar. When the cysts in the medium were exposed to Alb, some AlbSO and albendazole sulphone were detected in both cyst wall and cyst fluid, indicating that a part of Alb was metabolized by the cysts. In another experiment cysts exposed to the drug for 2 h were transferred to the medium without the drug for another 24 h. The release rates of the 3 drugs from the cysts were alike. In 1-2 h after transfer about 65-70% of the drugs absorbed by the cysts previously were released. Twenty-four hours after exposure the release rates increased to 75-85%, and the release of the drugs from the cyst wall was somewhat faster than that from cyst fluid.