Serum and plasma concentrations of thyroid hormones, catecholamines and cortisol as well as oxygen consumption were measured simultaneously in twenty patients in connection with cholecystectomy. Arterial blood samples were taken before surgery and at 2, 24, 48, 72 and 96 h after skin incision. A marked and prolonged decrease in T3 and free T3 concentrations was seen. A minor decrease in T4 was also observed but free T4 remained unchanged. TSH concentration was increased 2 h after skin incision, but at no other occasion. A marked increase in adrenaline and noradrenaline concentrations was observed 2 h after the start of surgery. This increase lasted 24 h for noradrenaline. There was a marked and prolonged rise in plasma cortisol after surgery. The oxygen uptake was higher than the preoperative value during the entire postoperative period. No correlations between thyroid hormones and catecholamines or cortisol or oxygen uptake were found. It is concluded that the most pronounced changes in thyroid hormones and TSH after upper abdominal surgery occur about 24 h postoperatively but catecholamines demonstrate their maximal increase immediately after surgery. The inverse correlation between T3 and catecholamines previously reported in burned patients, with their severe hypermetabolism, it is not present during the moderate hypermetabolism after abdominal surgery.