Determination of closing volume or phase IV reveals the closure of the airways when lung volume is reduced. But direct evidence of the airway closure or low volumes of these closed units has not yet been demonstrated. It seems reasonable to presume that residual volume (RV) would be underestimated if lung volume is allowed to keep at low level during its measurement due to the air trapped. RV was thus measured in a closed circuit rebreathing technique with O2 in low and high lung volumes in seven human subjects. Prior to rebreathing, lung volumes were adjusted by the tidal breath of O2 given as 15 per cent, 20 per cent and 90 per cent of the subject's vital capacity following maximal exhalation. This technique could not detect the closure of the airways in low lung volume as RV was found to be the same in low as well as high lung volumes.