Subacute sclerosing panencephalitis virus inoculated intracerebrally into suckling hamsters produced approximately a 70% incidence of morbidity and mortality and an 80% virus recovery rate in untreated control animals. A therapeutic regimen, consisting of poly I/C administered intracerebrally three hours before and 7 and 14 days following virus challenge, significantly reduced the incidence of early death and virus isolation. Delay of therapy for 24 hours after virus administration resulted in loss of therapeutic efficacy under the conditions of this experiment. Poly I/C administered intraperitoneally also was without therapeutic benefit.