Because wide swings in temperature can occur during cardiac anesthesia all patients undergoing cardiac anesthesia should have their temperatures monitered. This is especially true in situations where deliberate hypothermia during cardiopulmonary surgery is an area of controversy. This study of 20 cases of open heart surgery was undertaken to compare the changes in tympanic membrane, nasopharyngeal, rectal and great toe temperatures and of to evaluate their correlation during the induction, cardiopulmonary bypass, rewarming and post-cardiopulmonary bypass periods. The temperature at each site was monitored every 10 minutes for 60 minutes of each period. The results were as follows, During the induction period, the temperature of the tympanic membrane, nasopharynx and rectum decreased significantly(p<0.05~p<0.01), but the temperatures of the great toe temperatures increased for 20 minutes and then slowly decreased during the next 30 to 60 minutes. During the cardiopulmonary bypass period, the sympanic membrane temperatures which were best correlated with the nasopharyngeal temperatures(p<0.05~p<0.01), decreased faster than the rectal, nasopharyngeal and great toe temperatures. During the rewarming period, the tympanic membrane temperatures increased most quickly and were significantly correlated with the nasopharyngeal temperatures(p<0.05) only at 0 and 10 minutes. During the post-cardiopulmonary bypass period, the tympanic membrane and nasopharyngeal temperatures decreased slowly and were significantly correlated with each other(p<0.01), but the rectal and the great toe temperatures increased slowly.