The use of iodized oil 40 per cent has permitted the outlining of the bronchial tree in the living subject with great accuracy and detail. This suggested it as a possible agent in determining the caliber changes believed to occur in the bronchi in normal respiration. EXPERIMENTS Observations were carried out on living subjects—one cat, two dogs and five human beings. The animals were anesthetized with ethyl carbamate (urethane), and the oil was administered by tracheal puncture. In man, where no anesthetic was required, the films were taken in the course of diagnostic procedures. After injection of the oil, x-ray films were taken during the actual process of breathing, and in man also while holding the breath at the end of inspiration and at the end of expiration. The respiratory movements were recorded by a pneumograph held around the chest by a tape and attached to a tambour. The moment