In unanesthetized dogs, previously tracheotomized, the authors have found that an increase in intrapulmonic pressure markedly slows the heart for a long time. This inhibition is due to a reflex stimulation of the vagal center as well as to direct central asphyxia, since it is abolished by previous atropinization or previous double vagotomy. Marked increase in the intrapulmonic pressure renders a dog unconscious and anesthetic in 12-15 sec. The knee jerk disappears just prior to death. It is not so readily abolished by increased intrapulmonic pressure in the unanesthetized as in the anesthetized animal.