Backgrounds: End-tidal CO2 (PETCO2) monitoring is becoming one of essential respiratory monitoring systems during anesthesia. In this study, the differences between PETCO2 values measured from the 4 different sites were evaluated. METHODS: Healthy adult patients were studied (n=30). During N2O-O2-Enflurane anesthesia, PETCO2 was measured from the 4 possible monitoring sites, 3 from the breathing circuit and 1 from the monitoring lumen site of the specialized endotracheal tube connected to the distal endotracheal tube. After intubation, repeated PETCO2 measurements at 15mins (T15), 30mins (T30), 60mins (T60) and 90mins (T90) and ABGAs at T30 and T90 were done and the differences between arterial Pco2 and PETCO2 (P (a-ET)CO2) were calculated. In addition, to study the effect of changing fresh gas flow rate upon the PETCO2 values, PETCO2 measurements were done by varying the total gas flow rate from 4 L/min to 2 L/min to 6 L/min at T60. RESULTS: The Y-connector area (PETCO2- (1)) showed the lowest PETCO2 value, the elbow connector (PETCO2- (2)) and heat-moisture exchanger (PETCO2- (3)) areas, the intermediate, and the distal endotracheal site (PETCO2- (4)), the highest. The difference between the most proximal and distal sites was varied 2.4 to 3.0 mmHg and not statistically significant. PETCO2 values showed significant decreasing trend with time at each site (p<0.05). At T30 and T90, PaCO2 was not significantly different from PETCO2- (4) but significantly different from PETCO2- (1), (2), (3). The effect of changing fresh gas flow rate upon the amount of PETCO2 values of the different sites was not statistically significant. CONCLUSION: PaCO2 was significantly different from PETCO2 values measured from the breathing circuit sites but not significantly different from those measured from the distal endotracheal tube. It might be said that we have to pay special attention to these differences if we want to estimate real P (a-ET)CO2 difference.