In this study, we aimed to detect and quantify viral replication in living animals using two recombinant RVFV strains expressing either humanized version of the luciferase gene of Renilla reniformis (hRLuc) or enhanced green fluorescent protein (GFP) gene of Aequora victoria. Both RVFV viruses lack a functional NSs gene, which is a main factor of virulence in mice [33] , and are therefore avirulent in immunocompetent mice. However, in mice that are nonresponsive to type I IFN, the virus expressing either hRLuc or GFP caused lethality within 3 days, in agreement with previous data for the NSs-deficient Clone 13 [24] . In these mice, virus infection could be tracked by luciferase imaging in live animals and by the detection of GFP-positive cells from infected animals, by use of flow cytometry. We observed qualitative and quantitative differences in the in vivo tropism of RVFV in mice and show previously unsuspected sites of virus replication and modes of virus spread.