Two hundred and fifty three domestic rats (Rattus norvegicus and Rattus r. frugivorus) were experimentally infected with two different strains of Yersinia pestis ("Pexu-19" and "Rangel") by subcutaneous route. These strains were isolated from wild rodents from Northeast Brazil (Exu, Pernambuco State) and Venezuela, respectively. Thenceforth, they were maintained in culture and subinoculated into susceptible laboratory mice periodically. The survival of the infected rats varied from two to seven days. The animals which live longer were killed twenty days after inoculation. Three different inocula were used in each host (750- 4.250, 7.500 - 41.250 and 75.000 - 750.000 bacilli). the pathological lesions were studied in the liver, spleen, lungs and inguinal lymph node where the lesions were found to be more frequent. The liver lesions consisted of congestion, multifocal coagulative necrosis and acute inflammatory infiltration of portal tracts and sinusoides, with formation of microabscesses. In spleen there was a marked atrophy of the malpighian lymphoid folliches, as well as congestion, haemorrhages and acute splenitis with abscess formation. In lungs, acute pneumonitis was almost invariabby found, sometimes followed by abscess formation. The brazilian strain ("Pexu-19") showed a higher pathogenic power when compared to the venezuelan strain ("Rangel"). However, no difference was detected between the species used as experimental hosts since both of them showed a similar susceptibility to the two strains of Yersinia pestis utilized in the present investigation.