Objectives: Pediatric cardiopulmonary bypass involves the creation of a large obligatory priming reservoir. Packed red blood cells are an essential part of the cardiopulmonary bypass priming solution in children. The storage media in packed red blood cells might cause significant acid-base, glucose, and electrolyte imbalances, which have been associated with severe complications. The purpose of the present study was to evaluate the metabolic effects of fresh (≤5 days) versus old (>5 days) stored packed red blood cells added to the priming solutions of pediatric patients undergoing cardiac surgery. Methods: Blood samples were drawn from cardiopulmonary bypass priming of 30 consecutive pediatric patients undergoing cardiac surgery. Patients were divided into 2 groups. Fresh (≤5 days old) stored packed red blood cells were added to the priming solution in group 1, and old (>5 days old) stored packed red blood cells were added to the priming solution in group 2. In each group blood samples were drawn from the packed red blood cells on arrival to the operating room and from the priming solution immediately after packed red blood cells were added and after 20 minutes of prime circulation. Samples were also collected at the beginning of cardiopulmonary bypass and after 30 minutes. The last sample was collected on arrival to the pediatric intensive care unit. The levels of potassium, glucose, and lactate and the acid-base balance were analyzed in each sample. Results: There was a linear increase in potassium levels in packed red blood cell samples with increasing packed red blood cell age, ranging from 5.4 to 18.4 mEq/L. Significant differences in the concentrations of potassium, glucose, and lactate and the acid-base balance were found when comparing old and fresh packed red blood cells in samples taken during the packed red blood cell and early prime time. Those differences resolved after 20 minutes of reconstitution of the priming solution. The age of the packed red blood cells had no effect on the samples taken during bypass and those taken in the pediatric intensive care unit. Conclusion: The significantly higher concentration of potassium and lactate and lower pH in old stored packed red blood cells has a minimal effect on the final constitution of priming solution before and during cardiopulmonary bypass in children undergoing corrective cardiac surgery.