To investigate the motor control of the pulmonary airways we combined electrical stimulation of the cervical efferent vagus nerves of cats and dogs with measurement of pulmonary resistance (Rl) and anatomic dead space (Vd), and with anatomic study of rapidly frozen lungs. Stimulus frequencies between 1 and 12/sec produced almost the full range of Rl responses in eight cats. The major constrictor response to stimulation of one vagus nerve was in the lung of the same side in dogs and cats. A decrease of Vd associated with the increase of Rl in five cats during bilateral vagal stimulation suggested that the Rl change resulted from a generalized or large airway narrowing, rather than a local constriction. In seven cats whose lungs were frozen at end inspiration during unilateral vagal stimulation, the lobar bronchi on the stimulated side were smaller in diameter than comparable bronchi of the other side. In only two of seven cats was there definite constriction of the small cartilaginous and membranous airways of the stimulated side by the criterion of longitudinal mucosal ridges; in two there was slight constriction and in the other three, no change. Respiratory bronchioles and alveolar ducts of frozen sections were not constricted. pulmonary resistance; vagus nerve distribution; anatomy of pulmonary airways; vagus nerve impulse frequency; sympathetic nerve inhibition Submitted on May 1, 1964