According to the Brazilian Agricultural Ministry, the prevalence of cystic echinococcosis in the state of Rio Grande do Sul, Brazil has decreased between 1990 and 1998, possibly due to the common use of albendazole to control parasites in sheep. Therefore, albendazole (ABZ) action was tested on hydatid cysts of sheep experimentally infected with eggs of Echinococcus granulosus in Uruguaiana County (RS) with the aim of evaluating the observed changes in the parasite's prevalence. The sheep were divided into three groups of 15 animals each and treated as follows. Group 01 was a control (without treatment), groups 02 and 03 received 7.5 mg/kg and 15 mg/kg, respectively, of ABZ orally for 12 months, at intervals between 36 and 45 days. At the end of the studied period the animals were necropsied and the viability of the hydatid cysts was evaluated. Lung and hepatic cysts of sheep treated with ABZ were reduced in size. Degeneration and evident calcification were seen in 87.3% (group 02) and 75.7% (group 03), which were significantly different (P < 0.01) from the 17.3% demonstrated in the control group. It could be concluded that ABZ used early and over long periods, even in small doses, is capable of promoting lesions that devitalize the cysts. This leads to a decrease in fertility of E. granulosus in sheep and could be the reason why the prevalence of cystic echinococcosis has decreased in recent years.