Aim: The research paper presents the characteristic of cytoarchitectonics of the thymus of intact white mature male laboratory rats. Topicality of the study is due to the need to clarify the data on the contribution of each type of thymus cells in the formation of its structure. The aim of the research was to determine the specifics of localization and ultramicroscopic structure of thymus cells in male mature Wistar laboratory rats. Material and Methods: The study was conducted using histological and ultramicroscopic methods on 10 mature male laboratory rats, weighing 130-150 g. Semi-thin (0.5-1 μm) and ultrathin (0.05-0.2 μm) sections were made on a microtome UMTP-4 (Ukraine), which were stained with 1% methylene blue solution with the addition of 1% sodium tetraborate solution. Histological analysis and photographic recording were performed using Olympus light microscope (Japan) and DSM 510 camcorder with magnification in 1000 times. Results: With a detailed study of the semi-thin and ultrathin sections in the thymus lobules the specifics of localization and ultramicroscopic structure of thymus cells were clearly identified. The features of localization and ultramicroscopic structure of epithelial, mesenchymal, vascular and hematopoietic thymus cells were determined from the point of view of their functional loads and interactions. Conclusion: The described structural peculiarities of the components of the thymus and their relative location in different zones reflect significant organ polymorphism, which must be taken into account in order to achieve the required level of objectivity in the result evaluation of simulated biomedical experiments.