Suspension of brain fragments obtained from a patient with subacute sclerosing panencephalitis (SSPE) were inoculated intracerebrally in two rhesus monkeys. The animals remained apparently well for more than one year after inoculation. Signs of neurological involvement (paralysis of the posterior limbs with apathy and cachexia) appeared respectively 21 and 22 months after inoculation. The animals were sacrified. At histological examination of the central nervous system marginal and white matter gliosis, and inflammatory perivascular cuffings were found; there was also proliferation of the perineural satelite glia with satelitosis in the brain stem. A discrete process of chronic myocarditis was observed on visceral examination. In one of these animals an acidophilic intranuclear inclusion was found. From these animals brain fragments were removed aseptically and inoculated intracerebrally in 4 others rhesus monkeys which, 2 to 3 months after inoculation, showed signs of central nervous system involvement similar to those observed in the first passage animals with a shorter incubation period. They were also sacrified. The histological examination showed lesions, similar to those found in the animals of the first passage, increased with cortical spongiosis. The lesions observed in the animals from the first passage and which were also present with more intensity and shorter incubation period in the second group of animals inoculated, seem to be produced by the so-called "slow viruses".