The ventilatory mechanics of freely moving Caiman crocodilus were studied by cinefluorescopy and electromyography. The buccal oscillations serve only to flush the internal nares in olfaction. Ventilations are coincident with abdominal oscillations. The larynx ordinarily lies adpressed to the internal nares so that the posterior buccal chamber is excluded from the path of air flow during ventilation and does not contribute to respiratory dead space. The pulmonary pressures may be variably polyphasic and the tracheal flows diphasic. Exhalation involves an anterior shift of the liver by action of the transverse abdominal muscles, while inhalation proceeds due to contraction of the diaphragmatic muscle pulling the liver caudad. The various costal muscles facilitate air flow by shifting the position of the ribs. They also play a role in fixation of the flexible rib cage so that it resists the aspirating and compressing actions of the hepatic piston. The pattern of muscular activity shifts as the trunk is immersed; expiration becomes passive and inspiration requires increased muscular effort. The ribs, instead of changing position with each breath are comparatively fixed by the costal muscles, while changes in the volume of the pleural cavity are caused almost exclusively by movements of the hepatic piston.