Partial or complete deficiency of the clinical crown of non-vital teeth leads to a certain degree of deficiency of the dental arch, and to a certain extent disturbed functioning of the masticatory system. In such cases it is necessary to construct posts and cores to strengthen the tooth abutment as the base for crowns and bridges. The aim of the investigation was to compare the resistance of nonvital teeth, fitted with cast metal and prefabricated posts and cores, to dynamic loading in a high frequency pulsator, with special attention to different widths of root canal preparation. The results of the fractographic analysis show that the greatest resistance to dynamic loading was found in teeth with narrow prefabricated posts and cores, which endured loading of 1000N for an average 7,497,318 cycles, without the occurrence of fracture. Widening of the root canal in the case of preparation of the intraradicular part of the tooth for posts and cores, of more than 3.5 mm, significantly reduced its retention and increased the possibility of root fracture. All fractures occurred in the tooth root, while the posts and cores retained intact.