The aim of this investigation is to present the proportion of serotonin positive mast cells in the different layers of bronchial tree. According to Directive 2010/63/EU of the European Parliament, after anesthesia, 6 male adult Wistar rats were perfused with 5% paraformaldehyde in phosphate-buffered saline. Material from the lungs was removed and processed in accordance with the histological requirements for immunohistochemistry. Six μm sections were treated with Monoclonal Mouse Anti-Human Serotonin, incubated for one hour with DAKO-REALTM En-VisionTM detection system and visualized with diaminobenzidine. Demonstration of mast cells was done with toluidine blue staining on the same sections. For statistical analysis GraphPad Prism 6 for Windows was used. Most serotonin positive mast cells were observed in the tunica adventitia and tunica muscularis of the bronchi but less mast cells were seen in the bronchial lamina propria. The findings were similar in the bronchioles. Comparing the bronchi and the bronchioles, it was observed that the number of serotonin positive mast cells was higher in the larger ones.