Little is known about the replication of Karshi virus, a member of the mammalian tick-borne flavivirus group, in its rodent hosts. Therefore, we developed a quantitative real-time RT-PCR assay and measured the amount of viral RNA in selected tissues of infected Swiss Webster mice. Two-day-old mice were highly susceptible, with 100% fatality 9 to 12 days after infection, whereas infection in 9-day-old mice was less virulent, with death occurring only rarely. In nearly all cases, mice inoculated when 2 days old contained similar numbers of viral genome equivalents from blood and liver samples from any given mouse, with titers declining after day 7. In contrast, the amount of viral RNA in the brain began to rise rapidly 4 days after exposure, peaked at about 6 days after virus exposure (titer of > 10(13) genome equivalents/g), and remained at that level until euthanasia or death. Viral profiles were similar in needle-inoculated or tick-exposed mice.