Fil: Mills, James N. National Center for Infectious Diseases. Division of Viral and Rickettsial Diseases. Special Pathogens Branch; Estados Unidos.Fil: Schmidt, Karina. National Center for Infectious Diseases. Division of Viral and Rickettsial Diseases. Special Pathogens Branch; Estados Unidos.Fil: Ellis, Barbara A. Centers for Disease Control and Prevention. Coordinating Office for Terrorism Preparedness and Response. Office of the Director; Georgia, Estados Unidos.Fil: Calderon, Gladys. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Virales Humanas; Argentina.Fil: Enría, Delia A. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Virales Humanas; Argentina.Fil: Ksiazek, Thomas G. National Center for Infectious Diseases. Division of Viral and Rickettsial Diseases. Special Pathogens Branch; Estados Unidos.Prevalence of antibody reactive with Sin Nombre hantavirus (SNV) was evaluated from rodents captured over 31 months (March 1988 to September 1990) from six mark-recapture grids on the central Argentine Pampa. The most frequently infected rodents were: Akodon azarae (31/459), Necromys benefactus (8/141), and Oligoryzomys flavescens (10/281), which are known hosts of Pergamino, Maciel, and Lechiguanas hantaviruses, respectively. Relative population density and antibody prevalence varied seasonally and from year to year, population densities were highest in fall and prevalences were highest in spring. A positive association between antibody prevalence and body weight corroborated findings from other studies suggesting that hantaviruses are maintained in reservoir populations by horizontal transmission. In two of three host species, transmission was more frequent among male than among female mice. We found no evidence for a detrimental effect of hantavirus infection on host body weight, growth, longevity, movement, or reproductive preparedness. This analysis, based on cryopreserved specimens, represents the earliest conducted longitudinal, mark-recapture study of the dynamics of infection of autochthonous American hantaviruses in their sigmodontine host populations.