The mechanism of the cardio-acceleration elicited by electrical stimulation of the parahypoglossal area in the cat.

1. In chloralosed and in decerebrated cats localized electrical stimulation of a discrete parahypoglossal area (including the nucleus intercalatus and a discrete neighbouring region ventrolateral to the hypoglossal nucleus) elicited cardio-acceleration and arterial hypertension, either singly or together.2. The cardio-acceleratory response was decreased by intravenous administration of propranolol (2 mg/kg). In another group of experiments the response was abolished by acute bilateral cervical vagotomy but it was observed to return partially when stimulation was applied one or more hours after vagotomy. The hypertensive response was not affected by propranolol or by vagotomy.3. Evoked electrical activity with a latency of 3-10 msec was recorded from pre- and post-ganglionic sympathetic cardio-acceleratory nerves during stimulation of this region.4. These findings demonstrate the existence of a discrete medullary area which controls heart rate by activation of a sympathetic pathway and inhibition of a parasympathetic pathway.