The ability to estimate several sensations associated with breathing was studied in man by comparing physical stimulus intensity with the subject's judgment of magnitude. The data were expressed by the psychophysical power law, Ψ = K φ n, in which Ψ is apparent, or psychological magnitude, and φ is physical or real stimulus intensity. K and n are constants. The group mean value of n for pressure stimuli was about 1.5; for volume, about 1.3; and for ventilation, about 1.9. Refinements in experimental design to separate “active” ( i.e., subject generated stimulus by his own motor function) as compared with “passive” (observer generated stimulus) did not reveal significant differences in manner of assessment of pressure or volume (criterion was value of n), nor were pressure assessments of either kind affected by lung volume. The origin of the sensations probably resides in the chest wall.