Fifty-eight mice, chronically infected with different T. cruzi strains (Types II and III) were submitted to chemotherapy either with Nifurtimox (Bay 2502) or Benznidazole (Ro 7-1051). Twenty one mice were not treated and were used as infected controls. The duration of infection was from 90 to 400 days. Inocula varied from 1 x 10(4) to 5 x 10(4) blood forms. Treatment lasted for 90 days, doses being 200mg/kg/day during 4 days, followed by 50mg/kg/day for Nifurtimox and 100mg/kg/day for Benznidazole. Parasitological tests (xenodiagnosis, inoculations into baby mice and hemoculture) showed 85.3% negativation for Type II strains and 43% for Type III in animals treated with Benznidazole. As for Nifurtimox, there were 71.4% of parasitological negativation for the animals infected with Type II strains and 66% for those infected with Type III. IFA tests remained positive in 90% of treated and cured animals. Disappearance or marked regression of myocardial and skeletal muscle lesions was seen in the treated and parasitologically negative animals. The conclusion is that the treatment in the chronic phase of T. cruzi infection can result in parasitological cure in a high percentage of cases with regression of histopathological lesions, although with persistence of positivity of the IFA tests.