Objective - to study age-related changes in the topography and cytoarchitecture of the area X of the rat spinal cord at different stages of postnatal ontogenesis. Materials and methods. The study was carried out on pieces of the spinal cord taken at the level of the second thoracic segment of 65 female Wistar rats aged 3, 5, 7, 10, 15, 20, 30, 60, 90, 120, 150, 180, and 360 days (5 animals in each age group). The age-related changes in the square area of the lamina X, its neurons number per section, the density of neurons in the area of central canal and dorsal and ventral gray commisures were studied histologically on 14-mm-thick tangential cryosections stained by thionine according to Nissl method. Results. The area X on the transverse section of the spinal cord thoracic segment was clearly identified in 3-day-old rats. In the rat postnatal development, the area of the lamina X decreased, mainly due to the decrease in size of the dorsal gray commissure, despite the increase in the area of the ventral commis