Total body washout (hct less than 1%) in hypothermia was performed in 31 dogs using either a crystalloidal or a colloidal perfusate. Blood exchange and cooling was achieved by means of partial bypass and heat exchanger. Short lasting blood exchange for crystalloids without cardiac arrest resulted in 66% survival of the animals. When in addition circulatory arrest was established for 30 min at 14 degrees C oesophageal temperature blood exchange for crystalloids was not tolerated. All four animals of this group died within 19 hours presenting massive interstitial edema. Replacement of the crystalloidal perfusate by a colloidal solution (2.5 g% Dextran 60 at Ringer's Lactate) and establishing circulatory for 30 or 60 min resulted in survival rates of 71% and 50% respectively. The use of the colloidal perfusate effectively prevented edema formation. Death could not be correlated with the parameters controlled.