Major surgery evokes an endocrine stress response, characterized by increased serum cortisol, plasma adrenaline and noradrenaline. Furthermore, surgical stress is accompanied by lymphopenia and granulocytosis in peripheral blood. The changes in peripheral white blood cells have been demonstrated after surgery asiwell as after cortisol infusion. The aim of the present study was to investigate to which tissueslorgans peripheral blood lymphocytes are redistributed after major surgery. From 20 rabbits lymphocytes were isolated from peripheral blood, labelled with indium- 111-tropolene and reinjected intravenously into the rabbits. Ten of the rabbits underwent major surgery (upper laparatomy) during general anaesthesia, while the control group (n= 10) was anaesthetized without surgery. The endocrine stress response to surgery was measured as serum cortisol, plasma adrenaline and noradrenaline. The redistribution of lymphocytes was imaged with a gamma camera and calculated with a connected computer before, 2, 4 and 7 h after the skin incision. Compared to preoperative values, laparotomy resulted in an increase in serum cortisol from 116.6 to 461.9 nmol/l (mean) and a decrease in the fraction/percentage of lymphocytes in peripheral blood from 43.8% to 14.7% 7 h after surgery. Simultaneously, the activity of the heart and lungs together decreased to 76.1% of initial values, while the spleen activity was unaffected. The radioactivity of the lymphatic tissue increased to 137.8% and 134.7%, respectively, 4 and 7 h after the start of surgery. The results indicate that major surgery induces a redistribution of lymphocytes from peripheral blood to lymphatic tissue. It is suggested that the endocrine stress response may be of major importance.