Serotonin can accumulate in the adrenergic nerves of vascular tissues. We have determined whether in the isolated perfused dog saphenous vein 1) luminal administration of serotonin can result in its accumulation in the adrenergic nerves and 2) endothelium can interfere with the transport of the amine into the vessel wall. Saphenous veins were perfused with medium containing [3H]serotonin, [3H]norepinephrine or [3H]epinephrine; after washout, significant amounts of 3H were detected in the veins. The 3H-accumulation was augmented when the endothelium was removed mechanically; the augmented accumulation was only observed when the [3H]amines reached the tissues from the intimal side. In coronary arteries perfused with [3H]serotonin, similar results were obtained. No increased 3H-accumulation was noted in veins without endothelium perfused in the presence of cocaine. Nerve stimulation of veins labeled with [3H]serotonin caused an augmented release of 3H from the tissues without endothelium. Pargyline augmented the accumulation of [3H]serotonin and [3H]norepinephrine and decreased the difference between tissues with or without endothelium only for norepinephrine. Perfusion of venous segments with platelets, labeled with [3H]serotonin, resulted in a 3H-content which was significantly higher in the veins without endothelium. Our experiments show that serotonin and other amines, applied luminally to perfused blood vessels, can accumulate in the adrenergic nerves and that the endothelium can reduce this accumulation. Serotonin, originating from aggregating platelets, can penetrate the vessel wall much easier at sites of endothelial denudation and this serotonin also can enter the adrenergic nerves.