An electroencephalographic (EEG) study eith a multi-channel digital record was carried out to investigate EEG changes and their spatial characteristics accompanying hypothermia and cerebral ischemia during open-heart surgery in children. Subjects consisited of 10 children, ranging from 1 year 6 months to 4 years 6 months old, who underwent open-heart surgery with extracorporeal circulation and hypothermia. The EEG changes were evaluated visually and analyzed using a computer. (1) A discontinuous EEG pattern was notted during cooling in addition to commonly observed finding inculuding a decrease in fast waves, slowing and amplitude reduction, and was considered caused by rapid and transient suppression of cerebral function. (2) Power spectral and topographical analyses revealed a prominent decrease in fast waves and a relative increase in slow waves during cooling. Durong cerebral ischemia, in contrast, an actual increase in slow waves and decrease in fast waves were observed. (3) We noted the characteristics of bilateral synchronous rhythmic high-voltage slow waves among various types of transient EEG abnor-malities. (4) EEG changes induced by hypothermia differed by brain region. (5) EEG changes induced by hypothermia were influenced not only by body temperature itself but also by the rate of change in body temperature.