The authors describes the histopathological changes found in the central nervous system of two patients with subacute sclerosing panencephalitis. These lesions are compared to those found in the central nervous system of seven rhesus monkeis that had received inoculations of nervous tissue from the two patients. After an incubation period of approximately 18 months, the monkeys presented signs of damage to the nervous system, characterized by paralysis of the hind limbs and cachexia. Two of the animals succumbed to intercurrent pulmonary infections, before presenting any neurological manifestations. The incubation period shortened progressively after repeated inoculations from one animal to another; attaining, finally, a period of approximately 40 days. The Histopathological changes found in the two patients consisted of focal leptomeningitis, areas of neuronophagias, granulomas in the cortex and basal nuclei, marked loss of nervous with areas of status spongiosus, perivascular infiltrates, and gliosia of the while matter with no destruction of the myelin sheaths. In the experimental animals the histopathological alterations were similar, but less intense. The authors believes that the clinical and anatomical syndrome of subacute sclerosing panencephalitis may be caused by the measles virus, by viruses of the papova group, and possibly by viruses of other groups as well.