The effects of carotid body chemoreceptor stimulation by NaCN on respiration and phrenic nerve activity were studied in intact and vagotomized rabbits. In intact animals, an intracarotid injection of 30 micrograms of NaCN resulted in an elevation of phrenic nerve activity and a rapid onset of respiratory excitation associated with an increase in respiratory rate and the response was markedly potentiated after vagotomy. The change in respiratory rate was primarily due to a decrease in expiration time in intact animals, whereas it resulted from a pronounced decrease in inspiration time in vagotomized animals. The results suggest a suppressive effect of the vagus nerve on carotid body chemoreceptor reflex and a possible interaction of the latter input with the pneumotaxic center. An induction of a continuous increase in phrenic nerve activity accompanied by apneustic respiration by intracarotid dopamine was another evidence to support the assumption.