Nature and stability of semantic structures of some notions ("tree", "winter") have been studied in healthy subjects and in patients with lesions in frontal lobes of the brain by a method of notion definition (both active and passive variants were used). It has been established that in healthy subjects the structure of a semantic field consists of a core, and of a near and a far-away peripheries all differing in the number of feature-words they contain. In patients with lesions in frontal lobes there are more feature-words in the semantic fields of notions, however the core and the near periphery of a notion contain less significant words as compared with norm.