In respect of controls the beta-cells of the Islets of Langerhans in fed rats showed a significant decrease in nuclear volume after 40 min of anaesthesia with Thiobutabarbitone Sodium in a double-maximum-load dose. The alpha-cells and the cells of the exocrine pancreas do not show such alteration. The reduction in beta-cell acitivity is referred to a time and dose dependent effect of narcosis on the metabolism of the beta-cells and possible membrane phenomena. According to the morphological picture the otherwise missing signs of impaired function of the cells indicate a peripheral disturbance of glucose utilization during nacrosis. The blood-sugar levels support the proposed explanation.