A study of nuclear and nucleolar alterations found in cases of subacute sclerosing leucoencephalitis of van Bogaert (three cases) and of Dawson type (four cases) and in nodular panencephalitis of Pette-Döring (three cases) is reported. In all these types of encephalites there was found anisonucleolinosis with extrusion of nucleolinus, alterations not yet reported in these precesses. These nucleolar changes were found more frequently in Pette-Döring than in Dawson and Van Bogaert types, being a strong argument favouring the viral aetiology of these diseases.