This study included 17 young children, who were operated with cardioplumonary bypass for congenital heart defects and were cooled to 20 degrees C or 25 degrees C. No glucose, except for the pump prime solution, was administered during surgery. Samples of arterial blood, cerebral venous blood from the jugular bulb and mixed venous blood from the bypass circuit were obtained and analysed for concentrations of glucose, lactate and ketones as well as oxygen saturation. The prime content of lactate significantly contributed to the arterial lactate concentrations, which together with the cerebral arteriovenous (A-V) lactate differences remained elevated throughout the bypass period. The prime content of glucose had less influence on the arterial concentrations and these did not increase until the rewarming period, when indications of gluconeogenesis from lactate were found. Arterial ketone concentrations also increased during rewarming in parallel with significant cerebral uptake of ketones. The lowest cerebral A-V glucose, lactate and oxygen saturation differences were found at the target minimum temperature and this effect was significantly more pronounced in the patients cooled to 20 degrees C.