We tested the ability of tracheostomized, high-level quadriplegics to detect changes in ventilator-delivered tidal volume. Single breaths larger or smaller than control breaths were delivered, and the subjects indicated which breath was altered in a forced-choice procedure that minimizes the effect of subject bias. Quadriplegic patients detected changes in tidal volume of as little as 100 ml. Their ability to detect changes was comparable to that of a group of normal subjects similarly tested. These quadriplegic patients had little or no somatic sensation below the neck, and airways above the tracheostomy were not exposed to the stimulus. The quadriplegics consistently and emphatically reported that the sensation used in volume discrimination arose within the chest.