Spinally-projecting A5 neurons were studied with anatomical and electrophysiological techniques in the rat. A detailed study of the number and distribution of spinally-projecting catecholaminergic (CA) and non-catecholaminergic neurons present in a defined area of ventrolateral pontine reticular formation was performed using a sequential technique for the detection of CA fluorescence and retrogradely transported HRP. Using control animals and rats with 6-hydroxydopamine-induced lesions of spinal CA axons, it was concluded that up to 93% of all noradrenergic (NE) neurons present in the area investigated send an axonal process to the thoracic spinal cord and that NE neurons constitute at least 90% of all spinally-projecting neurons present in the same area. Single unit recordings of spinally-projecting neurons were obtained in the same area of the reticular formation in urethane-anesthetized, paralyzed and respirated rats. Based on the above-mentioned anatomical data, antidromic activation from thoracic spinal cord provided a necessary and sufficient criterion for the identification of A5 NE cells. These neurons had a conduction velocity of 2.5 m/s, a discharge rate of up to 4 spikes/s and all were inhibited by i.v. clonidine or desmethylimipramine (DMI). The inhibition produced by the latter drugs was always reversed by the alpha-2 adrenergic antagonists piperoxan or yohimbine. Antidromic (AD)-activation was followed by a period of inhibition whose duration was increased by raising the intensity of the stimulus or by administration of the NE-uptake inhibitor DMI. The effect of the latter was reversed by administration of the alpha-2 antagonist piperoxan.