The use is described of a composition comprising either a) alpha-linolenic acid (ALA, C18:3 n-3) or b) docosahexaenoic acid (DHA, C22:6 n-3) or c) DHA in admixture with eicosapentaenoic acid (EPA, C20:5 n-3), in a ratio of 1:0.5 to 1:1.7, respectively, and/or the pharmaceutically acceptable derivatives and/or precursors thereof; either a) or b) or c) being in a concentration not lower than 70% by weight of the total fatty acids weight in the composition, for the preparation of a drug for the prevention and/or treatment of the disturbances of the central nervous system (CNS) such as epilepsy, schizophrenia, bipolar (manic-depressive illness) and unipolar (major depression) psychiatric disorders, and by degenerative Alzheimer's disease and related forms of dementia.