The reflex cardiovascular depression caused by occlusion of the coronary sinus in the dog.

1. Brief occlusions of the outflow from the cannulated coronary sinus in the open chest anaesthetized dog caused a rise in coronary venous pressure, bradycardia, and systemic hypotension.2. The bradycardia and hypotension were directly proportional to the peak systolic coronary venous pressures above a threshold value of about 50 mm Hg.3. The bradycardia, but not the systemic hypotension, was abolished by intravenous atropine. Both responses were abolished by cutting the cervical vagi, and reversibly abolished by cooling them to less than 7 degrees C.4. During occlusions, there was inhibition of activity in single and small multi-fibre preparations of sympathetic efferent nerves in the ansae subclaviae and the cervical sympathetic.5. On the right side, nearly all afferent fibres ran in the recurrent cardiac nerve. There was no concentration of afferents in one cardiac nerve on the left side.6. It was concluded that the cardiovascular depression was reflex, and was caused by the stimulation of nerves or nerve endings in the ventricles of the heart.