1727 arterial blood gas samples of 341 cases of acyanotic congenitla heart diseases,79 cases of cyanotic congenital heart diseases and 88 cases of acquired heart diseases during cardio-pulmonary bypass surgery were analyzed.It was found that there were various degrees of acid-base disorders right before the bypass was on.During the speration,acid-base imbalance was aggravated because of anesthesia,fluid infusion,hypothermia,surgical manipulation,etc.Respiratory alkalosis was most common to occur,and metabolic acidosis was the secodn.After the cessation of bypass,acid-base disorders could be significantly alleviated or imporved.So it is belived that it is very important to monitor the changes of blood gases and acid-base balance and to manage timely the complications during cardiopulmonary bypass.Fluid infusion,administration of agents to improve the microcirculation.and replacement of alkaline electrolytes are helpful to maintain acid-base balance.