Samples of human lung were studied electron microscopically after fixation with glutaraldehyde and osmium tetroxide. Three different types of nerve profiles were found in the human lower respiratory tract. Type I nerve profiles contained small agranular vesicles 30-50 nm and large granular vesicles ranging from 60 nm to 210 nm. Type II nerve profiles contained small granular vesicles 30-50 nm and a few large granular vesicles ranging from 60 to 120 nm. Type III nerve profiles contained predominantly large irregular vesicles ranging from 60 to 210 nm, and only a few small agranular vesicles. The present electron microscopic results indicate that the smooth muscle layer of human bronchial tree receives adrenergic nerves.