Experiments designed to reveal an action of intercostal afferent stimulation on the rhythm of breathing were carried out on barbiturate-anesthetized, paralyzed cats, ventilated by a positive pressure respirator. Tetanic stimuli were applied to midthoracic, external intercostal nerves of intact animals and of animals displaying an apneustic pattern consequent to vagotomy and pontine lesions. In intact animals, the respiratory rhythm, as indicated by phrenic efferent activity, could be paced by intercostal nerve stimuli when they were timed to occur in a particular relationship to the lung volume cycle. Apparently, the action of the stimuli supplemented the phasic, volume-related afferent input. The response required considerable spatial summation and resulted from the action of intercostal afferents to either shorten inspiration or prolong expiration. These reflex effects could be attributed to activation of group II afferents. Although the actions of these afferents were relatively weak in the intact preparation, they proved potent in terminating an apneustic breath; brief stimuli, if they activated group II afferents, cut short phrenic discharge and initiated an expiratory phase, thereby restoring a reasonably normal respiratory rhythm.