ABSTRACT Changes in the configuration of the fowl syrinx during experimental stimulation of the extrinsic syringeal muscles were assessed by monitoring simultaneous changes in the airway resistance at a constant airflow rate. Stimulation of the caudal part of the tracheolateralis muscle produced a change away from the sound-producing configuration. The tympanum was drawn craniad, the external tympaniform membranes were stretched and airway resistance fell as the syringeal lumen enlarged. Stimulation of the sternotrachealis muscles produced a change towards the sound-producing configuration. The tympanum was drawn caudad, the membranes relaxed and airway resistance rose due to the constriction of the syringeal lumen. Tetanic stimulation of the cranial parts of both tracheolateralis and tracheohyoideus produced a marked retraction of the larynx and rostral part of the trachea. The threshold levels of air-sac pressure and airflow rate necessary to produce sound by passive ventilation of the respiratory system were monitored. Passive sound could be abolished either by stimulating the tracheolateralis muscle or by marginally increasing the tracheal resistance, simulating glottal constriction. Results are discussed in relation to current opinions on the importance of active and passive factors during vocalization in birds. It is concluded that the involvement of the extrinsic muscles may be necessary when producing low volume sounds by relatively weak physical effort. Passive factors appear to be more important during vigorous calling, such as crowing. In the latter case the main function of the extrinsic muscles may be to ensure the retraction of the larynx and changes in shape of the anterior respiratory tract.