A population of neurons localized to a small region of the brain stem reticular formation, the rostral ventrolateral reticular nucleus (RVL), is the principal area of the brainstem regulating resting, reflex, and probably behaviorally coupled control of arterial pressure. The critical area of the RVL reticular nucleus engaged in cardiovascular control surrounds a cluster of adrenergic neurons of the CI group, and is therefore designated the CI area. CI area neurons have a direct and extremely potent synaptic relationship with preganglionic sympathetic neurons of the intermediolateral (IML) nucleus of the spinal cord. Neurons in the CI area are tonically active and fire in relationship to the cardiac cycle, a rhythm imposed by baroreceptors. They thereby provide the background of excitation to preganglionic neurons to maintain normal resting arterial pressure. Which transmitter released by CI area neurons produces sympathetic excitation is uncertain, but it may be glutamate. CI area neurons are critical to the integration of a wide range of cardiovascular reflexes, including the arterial pressure responses to arterial baroreceptor, chemoreceptor, and other cardiopulmonary receptor stimulation, to pain and possibly to exercise, to brain stem ischemia and distortion, and probably to the arterial pressure elevations associated with emotional behavior. The cardiovascular neurons of the CI area are responsive to the actions of a number of neurotransmitters, many of which are confined to local circuit neurons in the region, including gamma-aminobutyric acid (GABA), catecholamines, acetylcholine, and several opiates. They also appear to be the site of action upon arterial pressure of a number of drugs‒including clonidine and possibly the β-blockers. Neurons of the CI area of the RVL reticular nucleus, therefore, appear to function as one of the most critical output systems of the brain for regulating arterial pressure in health and sickness. Am J Hypertens 1989;2:363S–374S