Fluorescence histochemistry and EM were used to study the structural consequences of in vitro exposure to the sympatholytic agent 6-hydroxydopamine on 2 blood vessels, the portal mesenteric vein and caudal artery of the rat. Depletion of catecholamines to indetectable levels was associated with clear signs of adrenergic nerve degeneration, such as cytoplasmic shrinking, virtual absence of dense core vesicles and swelling of mitochondria. All of the changes occurred within 2 h in the caudal arteries and 3.5 h in the portal veins. Comparison of electron and fluorescence micrographs of incubated control specimens with those of unincubated, fresh specimens showed that the nerve endings of the incubated controls were well preserved for at least 3.5 h. With destruction of nerve endings in such a short period of time, the processes of specific neuronal degeneration could be clearly demonstrated in isolated blood vessels.