ABSTRACT.Continuous EEG was recorded from 96 children and 9 adults undergoing deep hypothermia with or without a period of low perfusion or complete circulatory arrest. Hypothermia caused a progressive decline in amplitude and frequency and, in 41% of our cases, synchronous or asynchronous periodic paroxysmal activity. At the time of the anticipated arrest, which occurred at an average temperature of 16.3°C., 73% had persistent EEG activity. No EEG activity was seen within one minute after the arrest. Activity began to return with rewarming in a reversal of the pattern seen with progressive hypothermia. In patients without an arrest, activity had returned to normal by the time normothermia was regained. This process took longer in those with an arrest although the majority were back to prebypass baseline by the end of the procedure. The EEG changes were the same for children and adults.