Our previous study showed that the turkey pineal organ, in contrast to that of the chicken, is characterized by a follicular structure throughout the entire period of post-hatching life. Despite the preservation of the follicular organization, the histological structure of the pineal follicles in turkeys changes prominently with age. The present research was performed to investigate the cellular composition and organization of the follicle wall as well as the ultrastructure of parenchymal cells in the turkey pineal organ during the period of post-hatching development. Pineal organs were collected from female turkeys at 2 days, 2 weeks, 4 weeks, 10 weeks, 20 weeks, 30 weeks, 40 weeks, and 56 weeks post-hatching. The organs were prepared for immunocytochemical studies using antibodies against N-acetylserotonin O-methyltransferase (ASMT), glial fibrillary acidic protein (GFAP) and proliferating cell nuclear antigen (PCNA) and for ultrastructural examination. The results showed that regardless of age, the pineal follicle was formed by ASMT-immunopositive cells, among which rudimentary photoreceptor and secretory pinealocytes were identified. The second component of the follicle wall consisted of GFAP-immunopositive cells, as represented by ependymal-like and astrocyte-like cells. Rudimentary photoreceptor pinealocytes and ependymal-like cells formed the inner part of the follicle wall, while secretory pinealocytes and astrocyte-like cells created the outer part. Three forms of the pineal follicle structure characteristic of young (two days to ten weeks), young adult (20-30 weeks) and adult (40-56 weeks) turkeys were distinguished. These forms primarily differed in the relative dimensions of the inner and outer parts of the follicle wall. Ultrastructural studies showed prominent changes in the organization of rudimentary receptor pinealocytes during the investigated period of life. These cells developed until the age of 20 weeks, at which time they appeared as strongly elongated cells with a stratified, highly regular distribution of organelles. In adult turkeys, rudimentary receptor pinealocytes showed pronounced regressive changes; however, we never observed their transformation into cells of the secretory type. Secretory pinealocytes increased in number and size during the post-hatching period, which was especially pronounced after 20 weeks of age. The most prominent changes in the supporting cells included the intensification of GFAP-immunoreactivity due to the accumulation of filaments in the cytoplasm and the development of astrocyte-like cells. The increase in the number of secretory pinealocytes and astrocyte-like supporting cells resulted in the formation of two distinct parts of the follicle wall in the pineal organs of young adult and adult turkeys.