Many previous studies of laryngeal biomechanics using in vivo models have employed a constant air How source. Several authors have recently suggested that the lung-thorax system functions as a constant pressure source during phonation. This study describes an in vivo canine system designed to maintain a constant peak subglottic pressure (Psub) using a pressure-controlling mechanism. Increasing levels of recurrent laryngeal nerve (RLN) stimulation resulted in a significant rise in resistance followed by a plateau. For a given Psub, flow decreased significantly and precipitously with increasing stimulation and then quickly plateaued. Vocal intensity increased with increasing RLN stimulation until a peak was reached. After this peak, intensity dropped until a plateau was reached, corresponding to the flow minimum. At a given Psub, increasing levels of RLN stimulation resulted in a normal distribution of vocal efficiencies.