In order to study the relationship between catecholamine neurons and cerebral blood vessels, a technique was developed which permitted the simultaneous visualization of blue-green fluorescent catecholamine neurons and red fluorescent stained blood vessels in the brain of the normal rat. Sympathetic nerve fibers were found on the major arteries and in the pia-arachnoid at the base of the brainstem and also along paramedia and lateral perforating arteries as small as 10-12 microns within the brain. Running within the parenchyma, central catecholamine nerve fibers occasionally approached and intersected smaller blood vessels, either arterioles or venules of 8-12 microns, and infrequently climbed along or encircled these vessels for a limited distance, particularly within the lateral tegmentum. Across the nuclei of the brainstem, no overall contiguity of catecholamine terminals with capillaries was apparent, and no correlation between the density of catecholamine varicosities and that of capillaries existed. Only in regions with a high density of both catecholamine varicosities and capillaries, such as in the principal olivary nucleus, did a significant overlap of the two occur. But in most cases of moderately to densely innervated and vascularized regions, such as the solitary tract nuclei, the greatest concentration of terminals appeared over the parenchyma. Regarding the blood supply to the catecholamine neurons, their perikarya did not receive a particularly dense capillary supply relative to other nuclei. However, a special relationship of catecholamine cells to blood vessels was suggested, particularly in the case of dopamine neurons in the substantia nigra by the close apposition of cellular processes to adjacent small vessels. This morphological study was undertaken to determine whether central catecholamine neurons may significantly innervate cerebral blood vessels and accordingly, may function analogously to the peripheral sympathetic adrenergic neurons in the regulation of the vascular system. Although a limited number of associations between central catecholamine nerve terminals and small blood vessels suggested the possibility of an innervation in a few regions, the lack of an overall correspondence and correlation between the two across brainstem nuclei indicated that the analogy of central catecholamine neurons to the sympathetic nervous system was inappropriate. On the other hand, evidence of contact with vessels by presumed dendrites of the catecholamine neurons suggested a possible vascular sensory function.