Exchange transfusion is the standard mode of therapy for immediate treatment of hyperbilirubinemia to prevent kernicterus, but it has some complications. Potassium and acid-base variations are frequent in neonates who had exchange transfusion. These are directly related to factors such as type of blood, age of utilized blood, gestational age of the neonate, etc. In this study 47 neonates (42.6% boys and 57.3% girls) with blood exchange transfusion were evaluated over a period of 13 months in the University HospitalNo.1, Kerman Medical Sciences and Health Services. Blood samples of the neonates was collected from umbilical vein catheter, before and after exchange transfusion for K+ and blood gas measurement. Also one sample from blood bag was used for K+ measurement. The relationship between blood gas and factors such as gestational age, preservation time and volume of utilized blood and duration of exchange transfusion procedure were studied. In this study, like other studies, increasing in blood K+ concentration was found. This changes was obviously related to the preservation time of utilized blood and its K+ concentration. The potassium concentrations of utilized blood was often more than standard K+ concentrations for exchange blood (lower than 6 mEq/l) and was directly related to the preservation time of utilized blood. Sodium bicarbonate concentration of the neonates was obviously reduced after exchange transfusion and had a significant relation to the preservation time and sodium bicarbonate concentrations of utilized blood. Despite reduction in sodium bicarbonate, neonatal blood pH did not vary after exchange transfusion. After exchange transfusion the average PO2 and SO2 of venous blood was significantly increased. This increment was not related to the neonate's age, volume and preservation time of utilized blood, and its PO2 and SO 2. It is recommended that the preservation time of blood to be considered before transfusion.