To determine how closely end- tidal PCO2 measured by capnometer(Datex, Finland), a kind of infrared gas analyzer, reflects arterial PCO2(measured by Corning 175: U.S.A.) during general anesthegia, peak- tidal PCO2 and arterial were measured simultaneously. Thirty patients ranging between the age of 18 and 49, having no apparent abnomalities and having physical status class I by American Society of Anesthesiologist's classification-were seleted for the study. The anesthesia was induced with 2.5% pentothal sodium 4~5 mg/kg, succinylcholine 1mg/kg and incubated. The anesthesia was maintained with each 2 L/min gas flow of nitrous oxide, oxygenand 1 halothne. The patients were ventilated br anesthetic ventilator with tidal volume 8~10 ml/kg and ramie of 15 Per minute. The measurement of CO2 gas tension was performed 20 minutes after the induction when the patient's anesthetic conditions were stabilized. The CO2 gas ganlples were taken from mouth piece inserted between endotracheal adapter and circle breathing circuit, The arterial blood tramples were taken from the radial artery. There was a significant correlation between the end-tidol PCO2 and the arterial PCO2 in this series. The mean arterial PCO2 was 37.57±4.59 mmHg and the mean end tidal PCO2 was 23.73±5.78 mmHg. The mean difference between the arterial and the end tidal PCO2 was 6.53±2.23 mmHg. The correlation index between the two measurement was 0.8. In conclusion, the measurement of the end-tidal PCO2 by Datex Capnometer reflected the blood PCO2 and is convenient method of clinical use for its non invasiveness and continuous measurement of ventilatory status of patients under general anesthesia.