The development and pathogenicity of a Trypanosoma cruzi strain ("Chile 5") of low virulence were studied after infection of nude rats with different doses of blood trypomastigotes (10-10(7) parasites/rat). Peak parasitemias were correlated with the infection dose, which also influenced the mean survival times (26-36 days post-infection). Within 26 or 27 days, a subcutaneous injection of 10(7) blood trypomastigotes developed to about 8-20 x 10(7) parasites/ml.