Based upon the results observed in 15 patients submitted to tumor ressection by the CO2 microlaser technique and submitted to brain stem auditive evoked potential (BAEP) intraoperative determination, the authors emphazise the importance of the technique. BAEP during the intraoperative period and the ultrasonic control of removal of tumors proved useful in the 15 cases of brain stem tumors or tumor adherent to it. Morbidity and mortality are described, and the important changes in BAEP that precede bradicardia are detailed. The importance of advances obtained through BAEP in peroperative control in such surgeries is stressed.