Petr Nejedlý: Peripheral Units of the Old Czech Lexical System: Retreating Units (abstract of the thesis) The thesis deals with Old Czech lexical units retreating from the centre to periphery of the lexical system. It examines prerequisites of the process, i.e. loss of systemic relations of the lexical unit, and describes its individual forms (the extinction of a lexical unit in the last resort). The work submits an overview of both (intra)linguistic and extra-linguistic factors which are based on development of the lexical unit and influence its retreat to the periphery. Attention is paid to systemic changes in the language which modify internal relationships within the lexical system and also lead to the loss of central position of some lexical units.