Many studies have undertaken to determine the optimal volume of heparin diluted blood so- lutions to be aspirated from known internal volume of the arterial extension tube before blood sampling, in order to achieve acceptable blood gas values. This study was investigated the optimal volume of heparin diluted blood to be aspirated from various kinds of extension tube which volume is unknown. Blood gases were measured in samples of the radial artery or superior vena cava blood taken from 20 anesthesized patients who were taking elective or emergency surgery with indwelling catheter. Authors used 4 kinds of extension tube. There were 120 cm Cobe extension tube, 140 cm Cobe extension tube, 60 cm extension tube using I.V. set and 120 cm extension tube using I.V. set. Each of tubes was applied to five patients. Five blood gas samples were taken after withdrawing heparin diluted flush solution and blood from each extension tube. The withdrawing blood amounts is serially 0.5, 1.0, 1.5, 2.0, and 2.5 times of volume of each extension tube. And then another blood sample was taken from the 3-way stop-cock at proximal site of Cobe or LV. extension tube. Because this last sample was not diluted with heparin diluted flush solution, author assumed this blood gas values to be control group. Blood gas values of each samples were compared with control group. The difference between sample 3 and 4 was statistically significant(p<0.05) in case of small inner volume of extension tube. The difference between sample 4 and 5 was stastistically significant(p<0.05) in case of large inner volume of extension tube. So, authors concluded that as follows: If the inner volume of extension tube is small(volume <4 ml), blood gas samples should be taken after withdrawing 2.0 times of the inner volume of extension tube, however if the inner volume of extension tube is large(volume>4ml), blood gas samples should be taken after withdrawing 2.5 times of the inner volume of extension tube.