The pineal gland of rats exposed to the effect of low temperatures shows structural changes of a progressive nature: hypertrophy and hyperplasia of pineocytes, hypertrophy of nuclei and nucleoli;deprivation of followed by saturation by, pineocytes in sudanophilic granulations; intensified activities of nonspecific esterases, acid phossphatases and alkaline phosphatases; polymorphism and hypertrophy of mitochondria, hypertrophy of the Golgi zone, abundance of free ribosomes. On the basis of the results obtained indicating stimulated activity of the pineal gland, this organ is to be considered as integrated in the regulation system of adaptation to cold.