Factors affecting the tension on the external tympaniform membranes of the syrinx during respiration and vocalization were studied in 35 adult chickens. These membranes are controlled by the interaction of the sternotrachealis and the tracheolateralis muscles, tracheal air flow, and interclavicular air sac pressures to produce sound. The muscles are innervated by the hypoglossal nerves, with the left nerve playing the dominant role in innervating both the left and the right sternotrachealis muscles. Contraction of the tracheolateralis muscles applies tension on the external tympaniform membranes, holding them out of the tracheal air flow. The sternotrachealis muscles relax the membranes by pulling the trachea caudally. This allows interclavicular air sac pressure to push the membranes into the tracheal air flow which causes them to vibrate and produce sound. Section of the sternotrachealis muscles eliminates low level vocalizations and alters the character of calls at any intensity. Inactivating both pairs of muscles has less effect on vocalization because, without the tension caused by the tracheolateralis muscles the external tympaniform membranes are already relaxed, eliminating the need for the sternotrachealis muscles. Loss of pressure in the interclavicular air sac will stop vocalizations in chickens, even when the tracheal air flow is maintained at a rate adequate for calling. Tracheal air flow and interclavicular air sac pressure changes both result from the direct action of the respiratory muscles.