ABSTRACTInjection of serotonin (5-HT) into the left atrium or ventricle activates a hypertensive chemoreflex. The primary purpose of our study was to determine the afferent pathway(s) that mediates this response. A secondary goal was to localize the receptive sites of this reflex. We measured (1) changes in arterial pressure, (2) reflex vascular responses in skeletal muscle and paw, and (3) changes in renal nerve traffic that occurred after the left atrial or left ventricular injection of 5-HT. Injection of 5- HT (100 to 600 gg) into left atrium or ventricle produced large reflex increases in vascular resistance and sympathetic outflow. These responses were not reduced after bilateral cervical vagotomy. In separate experiments, increases in renal nerve traffic with left ventricular injection of 5-HT were assessed before and after cardiac sympathetic deafferentation. Interruption of cardiac sympathetic afferent pathways did not significantly attenuate increases in renal nerve activity with 5-HT. Injection of 5-HT (300 ulg) into the aortic root produced large increases in arterial pressure but this was not observed after injections into the vertebral or common carotid arteries or descending aorta. Injection of 5-HT (100 jig) into the left main coronary artery (perfused via a Gregg cannula from an external reservoir) resulted in a depressor reflex (Bezold-Jarisch). In contrast, injection of 5-HT (200 μg) into the left ventricle when the drug was prevented from reaching the left coronary artery produced a large pressor response. We conclude that the left atrial, left ventricular, or aortic root injection of 5-HT elicits a hypertensive chemoreflex response from receptors that receive their blood supply from arteries other than the major branches of the left coronary artery. The afferent limb of this reflex does not travel in cardiopulmonary vagal or sympathetic afferent fibers.