The selective lesion of the afferent serotoninergic system by means of intracisternal administration of 5,7-dihydroxytryptamine produces certain structural rearrangements in a part of cellular elements and in various parts of the neuropil of the neocortical parietal area. The deficiency of serotoninergic innervation influences the metabolic processes. This is evident from the change in the structure of protein synthesizing apparatus in a number of postsynaptic neurons. It is possible that the structural rearrangements observed in the serotoninergic and in the conjugated to it mediatory systems make the base of functional disorders in the neocortex and are accompanied with certain changes in the integrative activity of the brain.