Adrenal glands were collected from pigs of various ages under general anesthesia. Glutaraldehyde-fixed medullary tissue was postfixed with OsO4 for electron microscopy and with potassium dichromate or potassium iodate for light microscopy. Columnar epinephrine (E) cells formed cords between wide sinusoidal capillaries at the corticomedullary junction and were arranged in palisade fashion along the central vein and its major tributaries. The E cells usually were polarized, with the nuclei located away from the sinusoidal capillaries. Clusters of polygonal norepinephrine (NE) cells formed large central aggregates surrounded by E cells. Granulated vesicles were the predominant cytoplasmic feature of both E and NE cells. Round or oval E granules were bounded by a crenated membrane separated from the granule by a clear halo. The more electron-dense, elongate NE granules were bounded by a closely apposed, smooth membrane. The average longest granule axis was 270 nm for E granules and 305 nm for NE granules. Many cytoplasmic organelles were congregated in a granule-free paranuclear zone, which contained a prominent Golgi complex. Thin nonmyelinated nerve fibers (singly or in small groups) were interposed between the E and NE cells. Nerve fibers often were located close to the nucleus in a depression of the cell surface and often were wrapped by thin E or NE cell processes. The medulla of newborn pigs was composed predominantly or exclusively of NE cells. In both adults and pigs, E or NE cell cords radiated through the cortex toward the capsule, and isolated clusters of E or NE cells frequently were found in the capsule or zona glomerulosa.