The metabolic consequences of a "washed" cardiopulmonary bypass pump-priming fluid in children undergoing cardiac operations. The substrate load of the pump-priming fluid, especially glucose and lactate, has a major influence on the metabolic response of children during cardiac operations and may be detrimental neurologically. It is possible to cleanse the priming fluid of these excess substrates by ultrafiltration followed by the addition of a balanced electrolyte solution. We have used this technique in the bypass management of nine children over 1 year of age and weighing less than 18 kg who were studied from induction of anesthesia to 6 hours postoperatively. Frequent blood samples were taken for measurement of electrolytes, intermediary metabolites, and stress-related hormones. Throughout the study period electrolyte concentrations were maintained within the physiologic range and changes in blood glucose and lactate were minimized compared with those of matched historical controls from previous studies. During the period of cardiopulmonary bypass mean serum glucose concentrations varied between 4.4 and 7.7 mmol/L and peaked at 10.7 mmol/L just before skin closure. The mean serum lactate concentration did not exceed 2.3 mmol/L during the period studied. Thus ultrafiltration of the priming fluid and replacement with a balanced electrolyte solution results in the delivery to the patient of a reasonably physiologic substrate load, which is reflected in improved control of mean plasma intermediary metabolite concentrations. The method may form the basis of further clinical studies to determine specific aspects of the metabolic stress response in children. 