The effect on heart rate of distending the atrial appendages in the dog.

1. Stretching the atrial appendages in anaesthetized dogs caused an increase in heart rate. This response was unrelated to changes in arterial blood pressure and left and right atrial pressures.2. The increase in heart rate was a reflex response; the afferent path was in the vagi and the efferent solely in the sympathetic nerves.3. The receptors involved in this reflex have been localized to the atrial appendages but there is no conclusive evidence as to their nature.