Tilting the body in a foot-down direction from the horizontal produces changes in the resting end-expiratory lung volume which are essentially linear with the sine of the angle of the trunk to the horizontal. Tilting in the head-down direction produces slight and variable changes in this lung volume indicating that when the trunk is horizontal the functional residual capacity is at or near its minimal volume. Various factors which may influence the magnitude of the shift in the resting end-expiratory position are: 1) lung compliance, 2) compliance of diaphragm and abdominal wall with their attached structures, 3) compliance of rib cage, 4) effective length of the abdominal column which is considerably less than the total length of the abdominal cavity, 5) angle of trunk to horizontal. The greater the compliance of the lungs and diaphragm, the greater the angle of tilt, and the longer the effective length of the abdomen, the larger the external volume change will be. An increase in the compliance of the rib cage will increase the magnitude of the diaphragmatic shift but will decrease the external volume change. Because of this last, some patients may receive inadequate tidal volumes when tilted. In addition, children and infants who have a short effective length of the abdomen are even more likely to receive inadequate tidal volumes as a result of body tilting. Submitted on October 31, 1955