IT is well known that stimulation of arterial pressure receptors causes a depression of respiration1. Little is known, however, about the site of this depression, that is, whether it is effected on interneurones in the brain stem, on phrenic motoneurones, or on both. A clue to this problem was provided during an investigation of the central connexions of the carotid sinus nerve in the cat. In these experiments we have observed that a single maximal electrical stimulus to the sinus nerve can inhibit spontaneous activity in the phrenic nerve. Some of the properties of this depression of phrenic nerve activity are described here.