[3H]Praziquantel was absorbed rapidly by the cysts in mice infected with metacestodes of Echinococcus granulosus and treated with ig [3H]praziquantel 400 mg/kg (containing 3H 1.48 MBq). Within 24 h after treatment, 3H content in the ectocyst was 27.7-54.9% higher than that in cyst wall. The peak concentration of 3H in cyst wall and cyst fluid were 11.7-15.3% of those found in plasma. However, when the cysts were exposed to [3H]praziquantel 20 micrograms/ml (containing 3H 3.7 kBq), the 3H radioactivity in the cyst wall and cyst fluid reached equilibrium within 30 min-48 h after medication, corresponding to 45.5-63.3% of 3H radioactivity in the medium. Autoradiography showed that [3H]praziquantel entered into the germinal layer through ectocyst and laminated layer. The damage to germinal layer was related closely to deposition of the drug.