The postnatal development of adrenergic innervation was followed in peripheral blood vessels of Wistar rats. The femoral vessels and their branches, the superficial epigastric and saphenous vessels, and the tail artery, were investigated from birth to maturity. The proximal ends of the vessels were studied with fluorescence microscopy after the catecholamine was converted to a fluorophore by hot formaldehyde vapour, and ultrastructural morphology confirmed that the nerve varicosities mainly contained small vesicles with dense cores, typical of adrenergic innervation. Further confirmation was obtained with reserpine pre-pretreatment, the sodium borohydride specificity test, and experiments to alter the non-specific fluorescence of elastin. The nerves in the arteries were immediately adjacent to the external elastic lamina, and they retained this position throughout postnatal development. Of the three muscular arteries, the development of innervation was earlier and more intense in the saphenous and superficial epigastric arteries than in the tail artery. However, the tail artery surpassed the other two both in the total number of nerves and in the density of innervation per unit area beyond 12 days of age, and maintained this lead to maturity. The superficial epigastric artery had the smallest total number of nerves but had a greater density of innervation than the saphenous. The femoral artery did not develop any appreciable innervation. The femoral vein demonstrated the greatest amount of fluorescence of any of the veins, the others having considerably less innervation than their companion arteries.