Neuronal pathways in the peripheral nervous system have been traced using the fluorescent dye, Fast blue. Following implantation of a gelatin pellet containing the dye, or direct injection of the dye into the mesentery beside an artery, Fast blue is taken up by both nerve terminals and axons of passage, retrogradely transported by large numbers of sympathetic neurones and retained within the neurones for long periods of time without diffusion. Neurones projecting to both blood vessels of the mesentery and submucosa and to enteric ganglia of the segment supplied by the artery were found labelled in prevertebral and paravertebral ganglia as well as in ganglia lying along the major splanchnic nerves. Attempts to separate the vascular component from those neurones innervating enteric ganglia suggest that the latter are located in the prevertebral, coeliac and superior mesenteric ganglia and to a lesser extent in the splanchnic ganglia, while the vasomotor neurones are located in prevertebral, paravertebral and splanchnic ganglia.