Histochemical visualization of catecholamines and electron microscopy in the same tissue sample were used to localize and study catecholamine-containing nerve enlargements or swellings in male Wistar and Sprague-Dawley rat kidneys. These swellings lie in the perivascular nerve plexuses of arcuate and interlobular arteries near the points of origin of arterioles, and are composed of modified axons and associated Schwann cells. Transverse sections of the enlarged nerves reveal that individual axons are also enlarged, have processes or folds and make contact with one another. The axonal enlargements contain small mitochondria with a dense matrix and clusters of small vesicles, many of which are associated with an organelle composed of parallel cisternae of smooth membranes.