Previous work indicates that excitation of spinally projecting neurons in the rostral ventrolateral medulla evokes an increase in arterial pressure. The aim of this study was to test the hypothesis that axons of the ventrolateral neurons project to the spinal cord via a pressor region located in the dorsomedial medulla just rostral to the obex. In one group of experiments, a large quantity of horseradish peroxidase (HRP) was injected bilaterally into the spinal cord of rabbits following placement of a lesion unilaterally in the dorsomedial pressor area. This resulted in a much smaller number of HRP-labeled cells in the rostral ventrolateral medulla on the side of the lesion than on the intact side. In all other brainstem regions examined, however, the numbers of labeled cells were very similar on both sides. In other experiments, microinjections of sodium glutamate (which excites cell bodies but not axons of passage) were made into the dorsomedial pressor area. In contrast to the large pressor response evoked by electrical stimulation of this area, glutamate microinjections evoked a very small or no response. The results of both groups of experiments taken together suggest that the pressor response to electrical stimulation of the dorsomedial medulla arises from excitation of axons of passage which originate, at least in part, from the rostral ventrolateral neurons.