The development of endoscopic video systems and instrumentation has allowed therapeutic thoracoscopy for peripheral lung resection, pleural biopsy, open lung biopsy, closure of leaking blebs, parietal pleurodesis, and perieardiectomy and biopsy. The purpose of this study is to investigate the cardiovascular effects and blood gas changes during thoracoscopic surgery with one-lung ventilation and intrathoracic CO2 insufflation to collapse the affected site of the lung and so to get optimal surgical field under lateral position. Measurements of cardiac index, systemic vascular resistance, central venous pressure, heart rate, mean arterial blood pressure, and end-tidal PCO2 were done in twenty three patients who underwent bleb resection via thoracoscopy with intrathoracic CO2 insufflation. Each measurement was done 10 minutes after tracheal intubation(control), 10 minutes after position change, l0 minutes after one-lung ventilation, 30 minutes after CO2 insufflation, 1 hour ater CO2 deflation, and 6 hours after CO2 deflation. Also the measurement of arterial blood gas was done before anesthesia and used as a control. The results were as follows; 1) End-tidal PCO2 was increased after CO2 insufflation. 2) Arterial PCO2 was increased after CO2 insufflation and until 1 hour after CO2 deflation, and the value measured 6 hours afer deflation was recovered to the preanesthetic value. 3) Arterial PO2 was decrease after one-lung ventilation, but there was no change after CO2 insufflation. 4) Mean arterial blood pressure was increased after one-lung ventilation, after CO2 insufflation, 1 hour after CO 2 deflation and 6 hours after CO2 deflation from the control values respectively. 5) Heart rate was decreased after position change, and until one-lung ventilation, but it was increased after CO2 insufflation. 6) Central venous pressure was increased after position change, after one-lung ventilation and after CO insufflation. 7) Cardiac index waa increaeed after CO2 insufflation compared to the immediately preceding values. 8) Systemic vascular resistance was not changed throughout the procedure.