Measles virus isolated from the brain of a 12-year-old boy with subacute sclerosing panencephalitis caused a chronic, progressive encephalitis in experimentally infected rhesus monkeys. The infection was eventually fatal in spite of pre-existing measles immunity and a vigorous secondary antibody response in serum and cerebrospinal fluid of the infected animals. The findings provide a basis for studies into the pathogenesis and possible treatment of the human disease.