Mortality rates and viral replication in blood and brains of Wistar rats 6 h-26 days of age inoculated with 2 strains of Junin virus of different virulence were compared. Viral growth curves in brains showed no differences between strains. Differences in mortality rates were significant among rats 1-3 days of age. When the intracerebral (i.c.) route was used, high mortality rates were induced by the attenuated XJCl3 strain and low mortality rates were induced by the pathogenic XJ strain. When the i.p. or s.c. route was used, mortality rates were reversed: low for the attenuated strain and high for the pathogenic one. The use of different doses of each virus and the application of various routes of inoculation in 2 day old rats showed that 103 TCID50 [median tissue culture infectious dose] by the i.c. route resulted in the greatest difference in mortality rates.