Objective: Electroconvulsive therapy (ECT) is an effective and life-saving treatment modality, while its effects and side effects are still being discussed. In our study, we aimed to demonstrate the effect of O-2 administration on the cerebral oxygenation measured by Near Infrared Spectroscopy and the complications after ECT in the apnea period of ECT. Methods: This prospective, observational study was carried out with 40 patients whom ECT had been planned and who were undergoing ECT for the first time. Cerebral oximeter sensors and Bispectral index (BIS) monitoring were applied in all patients as well as routine monitoring. The participants were randomly divided into two groups. The patients who received administration of 6 L/min O2 through nasal cannula were included in Group O (n=20), and the patients who were not administered O2 were included in Group C (n=20). Hemodynamic data after induction, during the seizures and at the 10th min after the seizures, EtCO2, regional cerebral tissue oxygen saturation (rSO(2)), BIS values and complications after ECT were recorded in both groups. Results: No significant difference was determined between the groups with regard to rSO(2) alteration values measured during the seizures and demographic characteristics. Desaturation (SpO(2)<9O%) was observed in 12.5% of our patients. Of these five patients, four were in Group C, and one was in Group O. The EtCO2 values after the seizures was found to be significantly higher in Group C. The complication rate after ECT was also high in Group K. Discussion: Cerebral oxygen monitoring does not seem clinically useful in procedures with increased cerebral oxygen consumption, such as that in ECT in which the anesthesia time is short. In addition, it has been concluded that administration of oxygen during apnea may reduce respiratory complications that may occur during and after ECT.