Flow velocity of the right middle cerebral artery was studied in eight children during cardiac operations performed with profound hypothermia. Cerebral oxygen consumption was estimated by relating the difference in oxygen content between arterial and venous blood (jugular bulb) to flow velocity. In another six children, also during profound hypothermic procedures, the diameter of the middle cerebral artery was studied with an electronic echo-tracking instrument connected to a real-time ultrasound scanner. Flow velocity and estimated oxygen consumption decreased during cooling in proportion to the temperature decrease (r = 0.67, p less than 0.001, and r = 0.86, p less than 0.001, respectively), whereas the diameter was unaffected by temperature. At a nasopharyngeal temperature of 16.9 degrees +/- 1.9 degrees C flow velocity was reduced to 33.1% +/- 7.0% of the value obtained at 35 degrees C after induction of anesthesia. Correspondingly, the oxygen consumption decreased to 20.1% +/- 6.4%. The increase in oxygen consumption per 10 degrees C change in temperature was 3.6 (2.0 to 3.9) during surface cooling, 2.6 (1.9 to 2.7) during cardiopulmonary bypass cooling, and 2.7 (1.5 to 4.6) during rewarming. Flow velocity was not influenced by perfusion pressure during profound hypothermia within the range of 20 to 42 mm Hg (r = 0.14, p = 0.52) but was related to pump flow (r = 0.73, p less than 0.001). A pump flow down to 0.5 L/min/m2 was found to be adequate during stable profound hypothermia, as judged from the maintained high jugular bulb venous oxygen saturation (70% to 80%). It is concluded that flow velocity is reduced at hypothermia in proportion to the reduced metabolic rate, although modified by other factors that influence cerebral blood flow.