The individual fibers of the inferior laryngeal nerve and anastomosing branch of the superior laryngeal nerve arising from the upper five to six tracheal rings were studied. In 30 mongrel dogs using simple and multifiber nerve preparations, afferent nerve activity was studied both in the intact trachea and in an isolated segment of trachea with artificial stimuli of graded pressure and flow. The adaptation rate, threshold, spontaneous firing frequency, maximum firing frequency, and rate of change in afferent discharge were recorded. Subsequently, these fibers were electrically stimulated to determine what reflex effects could be produced. Afferent nerve activity recorded from the upper trachea was found to be sensitive to tracheal pressure changes of 0.5 to 12 mm Hg. Nerve activity paralleled the frequency of pressure changes. Electrical stimulation of these fibers demonstrated reflex bradycardia, bradypnea and alteration of blood pressure. These data and that of other investigators indicate that upper airway mechano-receptors may be significant, not only in the initiation of upper airway protective reflexes, but also in the regulation of normal respiration.