Changes of body temperature were observed in the esophagus and rectum by telethermometer during extracorporeal circulation in 40 cases of open heart for detection of significant differences in survivals and non-survivals. The following results were obtained; 1) Esophageal and rectal temperature(just prior to extracorporeal circulation) were 36.4±0.69 ℃, 36.8±0.8 ℃ respectively in non-survivals and 36.0±0.73 ℃, 36.4±0.8 ℃ in survivals after open heart surgery 2) Normal differences between rectal and esophageal temperature came to disappear after 30 minutes of extracorporeal circulation in two groups. 3) Esophageal temperatures were higher than rectal temperatures after extracorporeal circulation for open heart surgery in both groups. 4) Changes of temperature differences between rectum and esophagus have no correlation with mortality because of the variation in two areas in both groups.