Rhesus monkeys inoculated with Rift Valley fever (RVF) virus provide a model in which serial observations of serum viral antigen and antibodies can be made. In 9 non-fatal and 3 fatal infections, either antigen or IgM enzyme-linked immunosorbent assay (ELISA) antibodies were detected in every serum sample during the acute phase. Furthermore, viral nucleic acid could be detected by filter hybridization in most samples taken on days 1 to 3. Circulation of significant quantities of viral RNA provides an additional approach to the diagnosis and study of RVF.