Left ventricular reflexes have in the past been investigated in anaesthetised animals, generally using an open chest technique. We have studied the degree of bradycardia occurring during coronary arteriography in 200 patients with a view to localising the origin of the ventricular reflexes. We have correlated the decrease of sinus rate with the anatomical distribution and integrity of the coronary tree. The degree of bradycardia was not influenced by the origin of the sinus node or the AV node arteries, while there was a good correlation with the injection of contrast medium into the artery which supplied the inferior wall of the left ventricle. The occurrence of transient sinus arrest was also correlated with the injection into the same artery. The results suggest that the parasympathetic receptors are located mainly in the inferior wall of the left ventricle. This may well be the explanation for the clinical picture of bradycardia, hypotension, and peripheral vasodilatation often seen in acute inferior myocardial infarction.