Understanding the effect of vaccination on the body of chickens is impossible without exploring the morphology of the immune defence organs. The purpose of the study was to identify morphological changes in the Harderian gland of the Haysex Brown chickens aged 1, 15, 25, 50, 75, 100 and 120 days under the influence of vaccination. The following methods were used in the study: anatomical, histological, light-optical, morphometric, and statistical. Histological preparations of the Harderian gland of chickens of different ages (1, 15, 25, 50, 75, 100 and 120 days) of the vaccinated and unvaccinated groups were analysed. The features of the organ structure, its lymphoid structures and their types of forms were explored on histological preparations of the chicken Harderian gland. According to histological studies, it was established that this is a complex tubular lobular gland, which consists of secretory lobules with ducts. The interlobular connective tissue contains a significant number of lymphocytes, plasma cells, and other cells. In vaccinated chickens at 25 days of age, the presence of diffuse lymphoid tissue was noted, which was not observed in the control group, and at 50 days of age - the development of a nodular form of lymphoid tissue, the maximum number of which was observed at 100 days of age. In chickens of 75, 100, and 120 days of the age of the experimental group, the amount of lymphoid tissue in the Harderian gland increased, which was noted in the interlobular connective tissue and secretory lobules. It has been proven that the chicken gland, as a peripheral organ of immune defence, is developed at 50 days of age in both the control and vaccinated groups. The study of morphological changes in the Harderian gland of the poultry of different ages is an important tool for the development of preventive vaccination protocols in modern poultry farming. The results of the study should be considered when improving the technology of rearing and exploitation of chickens to ensure high vitality and, accordingly, productivity