Abstract <p>Regenerative endodontics is a strategy aimed at the regeneration of dental pulp or at least the formation of pulp-like tissue, ideally the pulp-dentin complex; regeneration of damaged coronal dentin, resorbed root, cervical or apical dentin. Regenerative endodontic strategies include stem cell transplantation and regenerative endodontic procedures (REP). REP methods are currently used in the endodontic treatment of open-apice teeth in patients aged 7–10 years. The most likely outcome of this treatment is root repair (“maturation”), rather than regeneration of its tissues. <b>Aim of the study:</b> Evaluate the efficacy of dental pulp stem cells in combination with fibrin gel for regenerative endodontic treatment of mature permanent teeth in a large animal model. <b>Materials and Methods:</b> The study was conducted on minipigs. While under general anesthesia (xylazine and tiletamine-zolazepam), the animals underwent an X-ray examination of their oral cavities. This examination was performed to select a tooth that most closely resembled a human tooth in terms of its position in the jaw arch, as well as the number and shape of its roots and canals. The first left two-rooted molar (P2) was selected. The crown pulp was amputated, and the root pulp was extirpated. The canals were then filled with fibrin gel containing allogeneic pig dental stem cells. The gel was mixed with fibrinogen polymerization catalysts (thrombin and calcium chloride) immediately before being injected into the canals for polymerization. Thus, the gel (3 μL, containing 15 000–18 000 cells per canal) took the shape of the canal during polymerization. Next, the gel was isolated with mineral trioxide aggregate, and the crown was filled with chemically cured, reinforced, packable glass ionomer cement. Four months after the endodontic treatment, the tooth was removed, fixed, decalcified, and used for a standard histological examination. Stepwise serial sections, 3–5 μm thick, were prepared and stained with hematoxylin and eosin. For quantitative assessment, seven sections were selected at different levels with a step of 10 μm, the images were scanned, and morphometric calculations were performed. Absolute and relative values of lengths and areas were assessed, as well as the degree of vascularization. <b>Results:</b> The length, shape and volume of the root canals of the first left premolar (P2) are similar to those in humans, which allowed us to use approaches and equipment developed for the endodontic treatment of human teeth, thus increasing the relevance of the model. Histological sections in the apical part of the canal showed the formation of vascularized pulp-like stromal tissue, and in the middle and apical thirds of the root, areas with a palisade layer of cylindrical cells—active odontoblasts producing dentin—were detected next to basophilic masses. There were no inflammatory processes in the periapical area of the endodontically treated tooth. No changes in the animals’ well-being or eating behavior were observed. <b>Conclusion:</b> The results of the pilot study indicate the prospects for the development of regenerative endodontics methods for the treatment of mature permanent teeth with formed roots. Scaffolds based on fibrin gel and dental pulp stem cells are generally suitable for the purposes of regenerative endodontics, but the method needs to be further improved to form pulp architectonics.</p>