The closure of small airways in gravity-dependent lung regions (airway closure) is thought to be an important factor influencing ventilation-perfusion (v/q) relationships. Airway closure has been noted by other authors as predisposing to atelectasis, under appropriate circumstances. We have assessed airway closure by a modified single breath nitrogen technique in ten normal male subjects in several surgical positions. Functional residual capacity (frc) was measured by a closed-circuit helium technique, to allow correlation of the airway closure findings with the alterations in lung volume associated with the posture changes. Functional consequences of airway closure are likely when the closing volume (the lung volume at which airway closure starts to occur) is abovefrc, with closure then occurring within the range of tidal breathing. Subjects were studied in the seated, supine, 15° head-down, and combined lithotomy plus 15° head-down position. Airway closure was found to involve increasing lung volumes with increasing subjects’ ages, in agreement with previous observations. In the seated position the age at which the closing volume exceededfrc was 49 years. This age decreased to 36 in the supine position, 35.5 head-down, and 32 in the combined lithotomy and head-down position. Body position, therefore, has an important effect on the critical relationship between the closing volume andfrc, and hence on the functional consequences of airway closure. *** DIRECT SUPPORT *** A00LO001 00003