In this paper we studied the classic and modern concepts concerning the structure, composition, origen and function of the nucleole particularly in relation to the neuronal cells. The materal of study consisted of a number of cases of human neuroviroses. A detailed description of the nuclear and nucleolar alterations verfied in the following diseases was made: Dawson's Subacute Polioencephalitis, van Bogaert´s Sclerosing Subacute Leucoencephalitis, Pette Döring's Subacute Panencephalitis, Acute Anterior Poliomyelitis and Rabies. The nucleolar lesions found consisted of early hypertophy followed by deep changes in its internal structure with a shape of vacuoles and granular condensations (the so called nucleolines) varying in number and size. Some of this deeply basophile granular corpuscles which show the same citochemical characteristics of the nucleoles are thrown in the charioplasma developing great spheroidal basophilic corpuscles. Commentaries about the nature of the phenomenon were made. Since such a finding is present only in virosis and in certain forms of intoxication and in genetic disturbances, its occurrence in an encephalitic of myelitic histopathologic pattern surely allows to admit its etiology as being due to a virus. From all the studied specimens the one which showed such nucleolar alterations mostly exuberant was the Pette Döring's Nodular Panencephalitis.