Infection of humans by viruses belonging to the families of Toga-, Bunya-, and Arenaviridae constitutes a major health problem worldwide and certain of the viruses have the potential to cause widespread epidemics. In the search for effective chemotherapy against these viruses several hundred nucleoside and nucleotide analogues have been screened for antiviral activity. Of the compounds tested, ribavirin, has been shown in laboratory animal models to have significant inhibitory effects against Rift Valley Fever virus, the bunyavirus Punta Toro, Hantaan virus and arenaviruses such as Pichinde, Junin, Machupo and Lassa Fever. Clinical studies with ribavirin in persons infected with Lassa Fever virus are in progress. Ribavirin or certain analogues have no detectable in vivo activity against arboviral encephalitic infections caused by Venezuelan or Western Equine Encephalitis or Semliki Forest Viruses. Other compounds including a series of triazolo derivatives, and the thiazole carboxamide nucleoside Tiazofurin appear to have in-vivo activity against certain of these exotic RNA viruses.