The neuranatomical location and pharmacological sensitivity of coeruleospinal neurons were studied with a combination of retrograde tracing experiments and single unit recording. Coeruleospinal neurons were multipolar, medium-sized cells and were found in the ventral division of the locus coeruleus and in the locus subcoeruleus. In the locus coeruleus proper, they presumably corresponded to the large cells of the ventral division defined in previous Golgi studies. Coeruleospinal cells were identified by antidromic stimulation from the cervical spinal cord. Their firing rate was slow and regular, their conduction velocity characteristic of unmyelinated fibers (0.65 m/sec). The method of antidromic stimulation also revealed that coeruleospinal neurones possess an anteriorly directed collateral traveling in the midbrain reticular formation outside the main noradrenergic dorsal bundle. These neurones were strongly inhibited by the iontophoretic application of morphine, noradrenaline, clonidine, GABA and excited by ACh. Although the coeruleo- and subcoeruleospinal neurones are clearly a group of cells distinct from the coeruleocortical projection, their electrophysiological and pharmacological properties are essentially identical.