Adrenaline-containing neurons of the rostral ventrolateral medulla (the C1 group) project selectively to autonomic spinal neurons in rats. Stimulation of these neurons electrically or chemically elevates arterial pressure, while neuronal blockade by microinjection of tetrodotoxin bilaterally drops arterial pressure to levels comparable to those produced by spinal cord transection. Adrenaline neurons of the ventral medulla appear necessary for maintaining normal levels of blood pressure, and thus may constitute a tonic vasomotor center.