The response to infection with Junin virus, wild strain Cba An 9446, and the antenatal and postnatal transmission of the pathogen in its natural host, Calomys musculinus, were studied. Intranasal infection in adult animals (90–120 days) did not produce mortality or illness during the 150-day period of observation. From day 21 to 150 after infection, 50% of the animals showed viral persistence with shedding of virus in both urine and saliva. The remaining half became seropositive, and no infectious virus was recovered from them. Although the virus did not infect fetuses during gestation, 50% of weaned pups nursed by viremic mothers were infected. Neither persistence nor immunologic response altered the reproductive pattern of the animals. The absence of reproductive failure in the infected host and the efficiency of postnatal transfer of Junin virus indicate that vertical transmission could contribute to the viral maintenance over tim