It has been reported recently that the genesis of phase IV (or closing volume) is not associated with the closure of the airways. The present study was undertaken to obtain direct evidence against airway closure in 4 normal male subjects. The single breath O2 test (SBO2 test) is extensively used to determine the residual volume (RV) and hence the closing capacity (CC). If this principle or technique of SBO2 test to determine RV is valid, a similar SBO2 test with only 10% of vital capacity (VC) as a tidal breath following maximal exhalation should cause an under estimation of RV, as the longer lobe will not receive any of the inspired gas due to estamination of RV, as the lower tube lobe will not receive any of the inspired gas due to closure of the airways. In this study, the SBO2 test was performed with various sizes of breath i.e., the lungs were inflated with O2 near 10% VC (as low lung volume), FRC and TLC levels, following maximal exhalation. In all cases, the computation of data from SBO2 tests showed no consistent changes in the magnitude of RV. The results strongly suggest that airway closure does not occur in low lung volume as the magnitude of RV is found to be the same r47w8 =mild with that of the high lung volume condition. The mechanism responsible for phase IV must be a phenomenon other than airway closure.