Blood flow distribution in tissues of mongrel dogs during hypothermia was studied with radionuclide-tagged microspheres. The animals were cooled at 21 degrees C and rewarmed under thiamylal sodiuni anesthesia. During hypothermia, cardiac output fell to 20% of the control; the highest rate of blood flow relative to normothermic values was observed in the subendocardium of the left ventricle, and the lowest in the hypophysis. Each tissue showed specific reactions to hypothermia. During hypothermia the myocardial and brain-stem blood flows were about 40% of the control; almost all of the digestive tract, striated muscle, adrenal gland, and hypophysis blood flows were maintained at 20% or less of the control. After rewarming, cardiac output recovered to values significantly lower than control. The myocardium, brain, renal cortex, and striated and smooth muscle recovered to control levels; however, blood flow to the digestive organs, bronchial artery flow to the lung, and flow to the endocrine organs did not completely recover by 2 after rewarming.