BACKGROUND AND OBJECTIVES Spectral electroencephalography has been used to measure anesthetic depth, hypnosis and sedation levels induced by different anesthetic agents. This study aimed at evaluating the effects of N2O on electroencephalographic (EEG) variables and sedation levels according to Observer Assessment of Alertness/Sedation (OAA/S) scale, as well as on hemodynamic and respiratory parameters. METHODS Thirty adult patients from both genders, physical status ASA I, aged 20 and 40 years, were submitted to the following protocol: after 10 minutes of relaxation, spontaneously breathing with eyes close, patients were given 30% and 50% N2O concentrations under face mask. Data were collected in three moments: M1- before N2O administration; M2 - 30% N2O in O2; M3 - 50% N2O in O2. Data for M2 and M3 were collected 15 minutes after stabilization of 30% and 50% N2O expired fractions (FeN2O), respectively, with a 5-minute interval between fractions. The following parameters were evaluated: electroencephalographic: BIS, SEF1, SEF2, power energy (PE) and burst suppression (BS); hemodynamic: heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP); respiratory: oxygen hemoglobin saturation (SpO2), carbon dioxide expired pressure (P ET CO2), minute ventilation and respiratory rate; clinical: patients were rated in a descending order (5, 4, 3, 2 and 1) according to OAA/S scale. RESULTS N2O administrated at 30% and 50% concentrations has statistically changed BIS, SEF1, SEF2, PE and OAA/S level of sedation in the studied moments. No clinically important changes were observed, as all patients were cooperative and slightly sedated, though SEF1 and SEF2 indications of deep hypnosis. Hemodynamic and respiratory parameters changes were not statistically significant. CONCLUSIONS N2O at 30% and 50% concentrations in non-premedicated patients has induced mild sedation according to OAA/S scale. There has been correspondence with BIS, but not with SEF1 and SEF2.