The measurement of pressure in the mouth 0.1 sec after the initiation of an occluded inspiratory effort (P0.1) has been proposed as an index of activity of medullary inspiratory neurons. If changes in FRC can be interpreted as important changes in the length-tension curve of the diaphragm or the total respiratory musculature, then changes in FRC from one occlusion pressure measurement to another can complicate such an interpretation of the P0.1 measurement. Forty-five subjects divided into three different groups were seated in a variable volume body plethysmograph. They had their FRC, P0.1, VT and VE measured while breathing air, 100% oxygen, 11% oxygen balance nitrogen, and 4% carbon dioxide in 20% oxygen balance nigrogen. All 45 showed a decrease in FRC during hyperoxia (-12%); 40 of 43 showed increases in FRC during hypoxia (14%); 42 of 43 showed an increased FRC during hypercapnia (15%). Changes in VE were small as were changes in P0.1 values. These latter changes generally followed the same pattern of changes as FRC though the magnitude of the changes showed more variability. We were unable to demonstrate a significant correlation between changes in FRC and changes in P0.1 under the conditions of our experiments.