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Departments of Cardiovascular Studies and Physiology, University of Leeds, Leeds LS2 9JT

1. In anaesthetized dogs, the activity from left atrial receptors with vagal afferent fibres has been examined during distension of small balloons in the pulmonary veins or the application of pulsatile pressures to an isolated perfused pouch of the left atrium.

2. Eleven fibres were found in nineteen dogs. The receptors discharging into these fibres were located, post mortem, to the atrial endocardium and reasons are given for believing that these fibres were myelinated and the receptors were B or Intermediate.

3. Distension of small balloons induced significant increases in the discharge of action potentials from the receptors, the increases were similar to those after infusions of saline or dextran.

4. In the perfused pouch the `dynamic' thresholds for pulsatile pressure were high as were the mean and pulsatile pressures required to induce bursts of action potentials similar to those observed from atrial receptors in the atrium of normal size. Restoration of the atrium to its normal size resulted in `dynamic' thresholds and responses to changes in pressure within the normal range. The changes could be explained assuming the Laplace relationship.

5. This evidence supports the conclusion from previous investigations in which these techniques were used, that the increase in discharge in atrial receptors with myelinated vagal fibres could be the cause of the increase in heart rate and urine flow.