SUMMARY The role of syringeal muscles in controlling the aperture of the avian vocal organ, the syrinx, was evaluated directly for the first time by observing and filming through an endoscope while electrically stimulating different muscle groups of anaesthetised birds. In songbirds (brown thrashers, Toxostoma rufum , and cardinals, Cardinalis cardinalis ), direct observations of the biomechanical effects of contraction largely confirm the functions of the intrinsic syringeal muscles proposed from indirect studies. Contraction of the dorsal muscles, m. syringealis dorsalis (dS) and m. tracheobronchialis dorsalis, constricts the syringeal lumen and thus reduces airflow by adducting connective tissue masses, the medial (ML) and lateral (LL) labia. Activity of the medial portion of the dS appears to affect the position of the ML and, consequently, plays a previously undescribed role in aperture control. Under the experimental conditions used in this study, full constriction of the syringeal lumen could not be achieved by stimulating adductor muscles. Full closure may require simultaneous activation of extrinsic syringeal muscles or the supine positioning of the bird may have exerted excessive tension on the syrinx. Contraction of m. tracheobronchialis ventralis enlarges the syringeal lumen and thus increases airflow by abducting the LL but does not affect the ML. The largest syringeal muscle, m. syringealis ventralis, plays a minor role, if any, in direct aperture control and thus in gating airflow. In parrots (cockatiels, Nymphicus hollandicus ), direct observations show that even during quiet respiration the lateral tympaniform membranes (LTMs) are partially adducted into the tracheal lumen to form a narrow slot. Contraction of the superficial intrinsic muscle, m. syringealis superficialis, adducts the LTMs further into the tracheal lumen but does not close the syringeal aperture fully. The intrinsic deep muscle, m. syringealis profundus, abducts the LTMs through cranio-laterad movement of a paired, protruding half-ring. The weakly developed extrinsic m. sternotrachealis seems to increase tension in the ipsilateral LTM but does not move it in or out of the syringeal lumen.