OBJECTIVE: To verify the incidence of T. cruzi transmission through breastfeeding during acute experimental Chagas' disease. METHODS: Fifteen female Swiss mice were mated and, after pregnancy confirmation, placed in individual cages. A few hours after birth, the females were inoculated with 0.1 mL of blood containing approximately 3 x 10(5) trypomastigote forms of Y strain of T. cruzi and continued breastfeeding for 25 days. RESULTS: In 142 offspring examined no infection through breast-feeding was observed. CONCLUSIONS: The low number of trypomastigote forms ingested by the newborn mice combined with biological and biochemical characteristics of blood trypomastigotes may explain the lack of transmission in this experiment.