1. The effect of ischaemia of the sino-atrial node on cardiac vagal action was studied in anaesthetized dogs.
2. The cut, cardiac end of the right vagus nerve was stimulated with a standard supramaximal stimulus every 10 s. The arterial supply to the sinoatrial node was occluded for periods of 1–3 min during this intermittent vagal stimulation.
3. Vagal action on heart rate was potentiated during ischaemia of the sino-atrial node.