In this study we investigated the effect of the antiretroviral molecule 2',3'-dideoxyinosine (Videx) against cerebral cysts in a murine model of toxoplasmic encephalitis caused by a wild cystic strain of Toxoplasma gondii. The role of the vehicle was also studied. Three doses were used: 50, 100 and 150 mg/kg of body weight/day. The doses of 50 and 150 mg/kg were prepared by dissolving pure 2',3'-dideoxyinosine powder in Maalox suspension before gavaging the mice; the dose of 100 mg/kg was prepared by grinding tablets of Videx that were suspended in water. A decrease in the number of cysts and a morphological modification of them were noted from day 15 with the lowest dose. The most important decrease could be observed with the dose of 100 mg/kg/d. After 30 days of treatment with this dose, 65% of the cysts were destroyed compared to controls. For the doses of 50 and 150 mg/kg/d prepared with Maalox, 36% and 51% of the cysts were destroyed respectively. So ddI has an effect on the cerebral cysts of T. gondii even at a low dose. The galenic formulation influences its action since the doses prepared with Maalox were less efficient than those prepared from ground tablets.