The kinetics of virus accumulation in various organs in mice inoculated with ZH501, rZH501-M847-A or rZH501-M847-G showed that RVFV replicates in the liver early in infection. The efficient replication of rZH501-M847-A and ZH501 in the liver and in other initial target organs probably induced high viremia titers that facilitated the spread of the virus to other organs, including the spleen, kidneys and brain ultimately leading to death of the animals. The virus titers in the liver of rZH501-M847-G-infected mice at 2 days p.i. were roughly ten times lower than those in the liver of rZH501-M847-A-infected mice (Fig. 4) . Likewise, virus titers in other organs and serum of the rZH501-M847-G-infected mice were substantially lower than those of the corresponding organs of rZH501-M847-A-infected mice. The present data imply that the level of virus titers in the initial target organ, e.g., the liver, at 2 to 3 days p.i. may determine the severity and ultimate outcome of RVFV infection in mice.