1) Plasma ADH levels measured by bioassay in the group with extracorporeal circulation were 2.3 +/- 0.6 muu/ml before surgery and 6.6 +/- 1.8 muu/ml during anesthesia. They increased to 196.5 +/- 62.3 muu/ml or about 100 times greater than before surgery during cardiopulmonary bypass. 2) In the group without extracorporeal circulation, plasma ADH levels were 1.5 +/- 0.9 muu/ml before surgery and increased to 44.1 +/- 15.2 muu/ml during operation. 3) After operation decrease in plasma ADH level was relatively rapid in both groups. It became three times that of the control level in the morning of the next day. 4) Marked increase in plasma ADH level during cardiopulmonary bypass was much the same as it was during hemorrhagic shock in dog experiments. 5) Fall in mean arterial blood pressure and loss of pulsatile blood flow will play main roles in this marked increase in ADH during cardiopulmonary bypass through stimulation of arterial baroreceptors and probably chemoreceptors. 6) In two cases with mitral stenosis, increase in plasma ADH during cardiopulmonary bypass was lesser than the other heart diseases.