Earlier work has shown an increase in serum magnesium to be a characteristic of hibernation with the magnitude of the change by species in this order: ground squirrel > bat > hamster. The present study describes another prominent change in serum electrolytes: namely, a drop in serum calcium with active and hibernating little brown bats exposed to the cold. Active bats and ground squirrels both appear to have a rise in serum potassium during cold exposure with less definite rise in hibernation. The hamster, as usual, shows a different response, in this case a decrease in serum potassium when active during cold exposure. Consistent with the other species, the hamster shows a slight rise in this ion in hibernation. On the whole, except for serum calcium in the little brown bat, and serum magnesium changes, the present experiments support the view that hibernation is characterized by the presence of serum electrolytes at control levels or slightly higher.