The intermediolateral cell column (IML) receives a dense noradrenergic (NA) input from the catecholamine-containing cell groups of the brain stem 6. This descending NA system plays a major role in the modulation of the sympathetic preganglionic neurons and hence may be of critical importance in the central control of vasomotor tone and reflexes4, iv. However, it is still not clear which of the catecholamine-containing cell groups project to the IML. Since the histofluorescence studies of Dahlstr6m and Fuxe 6, it has been widely assumed that the cells of origin of this projection are mainly the AI cell group of the ventrolateral medulla and possibly the A2 group of the solitary complex1, 12, 20. Recent autoradiographic studies demonstrate that the ventrolateral reticular formation projects to the IML but the biochemical nature of this pathway has not been analyzed 15. Thus, it is still not known if the AI cell group is responsible for this projection. Possible descending projections from the A2 cell group have not yet been studied. The locus coeruleus (A6 cell group) has been studied more thoroughly; on the basis of histofluorescencO 8 and autoradiographic 4 studies, there appears to be no evidence that the locus coeruleus projects to the IML. The A7 cell group, which lies in the region between the ventrolateral border of the brachium conjunctivum and the lateral lemniscus, appears to project to the spinal cord 6,~ 9, but the area of termination has not been identified. Using combined stains for horseradish peroxidase (HRP) and monoamine oxidase, Satoh et al. have provided evidence that the A5 cell group projects to the spinal cord 9, but the area of termination has remained unknown. This cell lies dorsal and lateral to the facial and superior olivary nuclei and medial to the root of the Vllth cranial nerve just prior to its exit from the brain stem 12. The purpose of this study is to report that the A5 cell group projects directly to the IML. First, we have demonstrated this projection by the anterograde transport of [all] amino acids. Second, we have found that the A5 cells were retrogradely labeled after HRP injections in the spinal cord and then demonstrated that these same labeled cells also showed catecholamine fluorescence. Third, we found that labeling of this pathway, either with autoradiography or with HRP, was abolished by pretreatment with 6-