To determine how intraoperative hypothermia associated with coronary bypass surgery (CABS) alters sympathetic nervous system (SNS) activity, we prospectively studied 21 adult CABS patients and measured preoperative, intraoperative, and postoperative circulating catecholamine concentrations. Because thyroid hormone levels change rapidly following CABS, we also serially measured these hormone levels. The measured plasma concentrations for each of the above variables were corrected for hemodilution during CABS by using serum albumin changes as a reference. It was concluded that important alterations in SNS activity and thyroid hormone homeostasis occur in humans during CABS and deep hypothermia, and that changes in core temperature may contribute to these findings. We speculate that these hormonal changes may influence the response to adrenergic receptor therapy in hypothermic patients and may contribute to arrhythmias during rewarming and the immediate postoperative period.