The nerve supply to the alveolar ducts and alveoli of mouse lungs was studied with the electron microscope. In the alveolar ducts, nerve bundles consisting of as many as nine unmyelinated axons surrounded by connective tissue fibers were usually located in the interstitium surrounding the openings of the alveoli. In the alveolar walls, nerve bundles consisting of as many as five unmyelinated axons were located in the interstitium between pneumocytes and the capillaries. The unmyelinated axons contained neurotubules and some mitochondria, and were partially or completely surrounded by Schwann cells. Two distinct types of enlarged nerve endings were identified. The first type contained many small mitochondria and was either associated with the type I pneumocyte or located alone in the interstitium. The second type was packed with numerous large dense-core vesicles and was in close contact with the type II pneumocyte. It is suggested that the first type of ending is sensory and the second type is motor in function.