The change of colloid osmotic pressure (COP) was measured during simple deep hypothermia in patients and in dogs. The following results were obtained; 1) There was no change in COP in dogs without occlusion of blood flow. This suggests that simple deep hypothermia has no effect on COP. 2) In the cases of open heart surgery with occluding blood flow for 40 minutes at 20 degrees C, COP remained almost constant during the cooling period. COP decreased from 23.0 +/- 0.8 mmHg to 19.1 +/- 0.8 mmHg after occluding blood flow and it did not recover with rewarming. 3) COP recovered during rewarming period with using methylprednisolone. 4) These observations suggest that the change of COP is caused by the change of microcirculation due to hypoxia during occlusion, and methylprednisolone is useful to prevent it.