Alzheimer's disease is the most common cause of neurocognitive disorders. The symptoms appear over time, throughout a number of months or years, and vary in their intensity and occurrence. The earliest common symptom is a disturbance in episodic memory with the exception of atypical presentations of the disease. Language impairments are part of the clinical picture of Alzheimer's disease. Language difficulties are first presented as anomic aphasia. As the disease progresses these difficulites appear at other levels of language processing, thereby developing into more severe forms of language disorder. Language processing depends on the integrity of neural networks which becomes disjointed as neurodegenerative changes start to affect a larger area. While lexical processing is very fast and precise, the underlying mechanism is extremely complex. Levelt’s model of speech production explains that underlying mechanism while assuming the possibility of issues arising at three different levels of lexical process – concept, lemma or lexeme. Furthermore, a number of psycholinguistic factors can affect lexical retrieval. The aim of this thesis is to examine the lexical processing on all three levels, while focusing on the aspect of Levelt’s model of speech production regarding the word itself, and to determine at which point the lexical processing becomes impaired in a participant suffering from Alzheimer’s disease. Lexical processing was examined with different tasks, which were modelled specifically for the purpose of this thesis and a few tasks taken from the CAT-HR test. Three psycholinguistic factors - imageability, frequency and word length, were controlled in these tasks. Qualitative analysis results clearly indicate the participant has difficulties in processing semantic and syntatic information, while lexeme processing remains intact. There are indications that the lexical process is impaired due to insufficient activation from the conceptual level, or possibly stemming from an impaired mental lexicon which delays access and retrieval of appropriate words during lexical processing. The impairment of processing on conceptual level cannot be fully ascertained as the patient also had difficulties in visual processing. In tasks which did not require visual perception, the patient did not show difficulties in processing conceptual information.