Pathological changes in alveolar hydatids from experimentally infected gerbils were studied after administration of albendazole or praziquantel. The animals were divided into several groups including the control one, and drugs of different dosages were given separately to different groups with diverse durations of infection. According to the microscopical observation, the hydatids could be categorized into 3 grades: grade I, degenerated cysts; grade II, stable cysts (showing no proliferation); grade III, proliferated cysts. Larger proportion of grade I cysts were found in the group treated with albendazole at high dosage (33.3%) and the two groups treated with praziquantel (25.0% and 16.3%). The proportion of grade I cysts in gerbils with shorter duration (45 d) of infection was larger than those with longer duration (60 d). Lymphocyte infiltration to various extent was present surrounding the alveolar hydatid cysts in all the treated groups. More marked infiltration of lymphocytes was observed in albendazole treated gerbils (106.9 +/- 34.3-300.1 +/- 59.5) (P < 0.001). However, the combined medication of albendazole and praziquantel was less efficacious.