Summary Following routine bronchography, cine-radiography has been used to study the movements of the trachea and main bronchi in man. The appearances seen on breathing and coughing are described and broncho-vascular movements noted. The changes in calibre throughout the trachea and main bronchi are considered to be passive and result from pressure changes. No evidence to support the peristaltic theory of smooth muscle contraction has been found during breathing and coughing. The function of bronchial smooth muscle is discussed.