The influence of vagotomy on resting vagal tone arterial Po2, the heart rate and blood pressure responses to different grades of hypoxia was studied in three species of anesthetized animals. Vagotomy without hypoxia produced a sustained increase in heart rate of dogs but not in rabbits or sheep. In the dogs, where this was studied more fully, heart rate showed sustained increase over several hours with no significant change in blood pressure or arterial Po2. It is concluded that there is species difference in the resting vagal tone during anesthesia. The relationships between graded arterial hypoxia with heart rate and blood pressure have been determined with and without intact vagus nerves. Before vagotomy hypoxia significantly increased the heart rate of dogs and sheep and reduced that of rabbits. Vagotomy decreased the bradycardia observed in rabbits, but did not change the response to hypoxia in dogs and sheep. The greater increase in heart rate of dogs calculated statistically and were due to the pre-existing resting vagal tone. After vagotomy, heart rate response to graded arterial hypoxia was increased rather than decreased in all animals.