The regular involuntary inspiratory muscle contractions that occur in normal conscious men during breath holding were quantified by means of the waves of intrathoracic pressure that they produced. A stream of cool air circulated through the nose and mouth reproducibly inhibited the contractions. The degree of inhibition increased with increasing nasal flow in the range of normal resting respiratory flow and with lower temperature of circulating gas. The effect depended on the phase of respiration in which flow occurred and was abolished by local anesthesia of the nose and pharynx. The results demonstrate that the upper airway contains flow-sensitive receptors, the discharge of which can have a marked influence on respiration.