In the present study swiss mice received the drug Ro 7-1051 (N-Benzyl-2-Nitro-imidazolacetamide) before infection with the Colombian strain of T. cruzi and results were compared with infection with the same strain but with treatment on the 7th day. Evaluation was performed by means of parasitemia, xenodiagnosis, inoculation in new-born mice and histopathological finfings. Results were better (55.5% of negatives) in pretreated animais as compared to animais treated on the 6th day following infection (28.5% cure rate).