Groups of mice were inoculated with six Trypanosoma cruzi strains and then treated with 32 mg/kg body weight allopurinol for 10 consecutive days. Effects of the drug on mortality rates were closely evaluated and repeated fresh blood examinations were done. Infected mice showed at least four parasitemia patterns with varied mortality rates and parasitemia levels. Evidence is provided that, independently of the parasitemia pattern or level and strain origin, there are marked differences in the sensitivity to allopurinol between the several T. cruzi strains studied. These differences in drug response seem to be related to biological characteristics of the strains and pose further challenges in the rational therapy of Chagas' disease.