[State of the epiphysis during winter hibernation]

It was found that the pineal gland of active ground squirrels (Citellus erythrogenys) differed from this gland in rats by a more pronounced differentiation into cortical and medullary zones, by larger pineocytes with polymorphous nuclei and numerous pseudokariosomes. Hybernation was accompanied by reduced concentration of serotonin in the pineal gland, by the disappearance of differences between the cortical and the medullary zones and by a number of morphological signs of functional depression. The appearing changes were similar to those occurring in denervation of the pineal gland.