1. With standard psychophysical techniques the ability of normal subjects to detect negative pressures applied at the mouth was estimated.
2. The ability to detect changes in pressure when confined to the upper airway by closure of the glottis was less than when the pressures were transmitted below the glottis and were actively overcome by inspiratory muscle contraction.
3. The ability to detect changes in pressure with the glottis open was also impaired by passive ventilation and vibration of the chest.
4. Our results imply that the most sensitive mechanism for the detection of negative pressures applied at the mouth involves afferent information from active inspiratory muscle contraction.