AbstractThe distribution and seasonal variations in the numbers of mast cells in the brain of 27 hedgehogs (Erinaceus europaeus Lin.) were investigated. Mast cells are numerous in thalamus and are mostly found in close proximity to blood vessels. Mast cells are frequently found in pedunculus cerebellaris, and they are always seen in the ependymal cell layer of the 4th ventricle. It was shown that the number of mast cells in thalamus is increased during hibernation and that the brains of females contain more mast cells than those of males. The phsyiological role of the increase in the number of mast cells during hibernation is discussed.