Gas trapped in the lungs of normal subjects breathing at functional residual capacity (FRC) was measured by a modification of the closed-circuit helium technique. “Closing volume” (CV) was also measured, by the single-breath nitrogen method. Volumes of as much as 193 ml, representing 10 percent FRC, were found trapped, and the amount trapped could be correlated with the relationship between “closing volume” and FRC. Significant volumes were trapped only when CV equalled or exceeded FRC. Compared with the sitting position, the supine position was associated with less FRC and a greater VTG when CV was greater than FRC. It is concluded that the results are evidence of the reality of airway closure and the detrimental effect on gas exchange of die supine position. Altered relationships between FRC and CV may cause the hypoxemia associated with anesthesia and that which follows abdominal surgery