Lexical decision task in an event-related potential experiment was used in order to determine the organization of mental lexicon regarding the polimorphemic words: are they stored as unanalyzable items or as separate morphemes? The results indicate the later: while monomorphemic words elicit N400 component, usually related to lexical-semantic processing, prefixed words and prefixed pseudo-words elicit left anterior negativity (LAN), usually related to grammatical (morphosyntactic) processes. These components indicate that the speakers apply grammatical (i.e., word-formation) rules and combine morphemes in order to obtain lexical meaning of the prefixed word.