Hibernators (Glis glis) were cooled to colonic temperatures near 0°C in an ice water bath within 11/2 hours during the summer and during the hibernating state. Thereupon cerebral Ha, K and H20 contents were determined. Such marked cooling during the summer produced in the brain of these animals an increase of H20 content by 0.61 ml/100 g fresh weight and of the sodium content by 10.29 mval/kg dry weight while the potassium content simultaneously rose by 29.53 mval/kg dry weight. During the winter the water content remained unchanged and the sodium content decreased by 5.12 mval/kg dry weight and the potassium content by 23.69 mval/kg dry weight. When cardiac activity was artificially interrupted in the winter at body temperatures below 10 ° C there was again an increase in H20 by 1.02 ml/100 g fresh weight, of sodium by 29.21 mval/kg dry weight and of potassium by 19.19 mval/kg brain weight. Thus, during the summer hibernators develop cold induced brain swelling at such low body temperatures, which does not occur during the winter with the circulation intact. This cold swelling is thought to be caused by an inhibition of active cation transport. This has been discussed in relation to earlier results on hypothermie rats and dogs [3,10,14].