Abstract. Clinical observations indicate that hypothyroidism negatively affects the structural and functional state of the body as a whole, including the immune system. In the body of patients with hypothyroidism, various immune disorders develop, which depend on the level of thyroid hormones and the duration of the disease. To date, the structural features of the immune system organs, including Peyer's patches, of offspring born from mothers with thyroid hormone deficiency have not been sufficiently studied. The aim of the study was to elucidate the effect of experimental maternal hypothyroidism on the cellular composition of various structural and functional zones of Peyer's patches in early postnatal ontogenesis. Hypothyroidism was induced in white outbred female rats weighing 180-200 g by administering mercazolil per os at a dose of 5 mg/kg for 21 days. The PB of the offspring born from them was studied using morphological, morphometric and statistical research methods. It has been established that under experimental hypothyroidism in animals there is a decrease in the mass of lymphoid organs and their cellular composition. Quantitative shifts in various PB zones progressed on the 21st day of experimental hypothyroidism and manifested themselves as a decrease in the number of cells in all structural and functional zones, mainly due to lymphocytes, macrophages, plasmacytes and destructive cells in all PB zones. In all zones, the total number of cells showed a pronounced tendency to decrease.