The lungs of rat fetuses at various stages of gestation and lungs of infant rats were examined histochemically for acetylcholinesterase (AChE) and norepinephrine (NE). No AChE is present in the fetal lungs until 15 days of gestation. At this stage a number of large round cells appear which stain heavily for AChE. These cells disappear by the 18th day of development and at 18 days no AChE-positive structures are demonstrable within the lung. The large AChE-positive cells are of similar size and distribution to fluorescent cells which become apparent after treatment of the mothers with L-DOPA. At 20 days, the day before delivery, a diffuse AChE reaction appears in the walls of large branches of intrapulmonary bronchi. At 20 days, also, sparse NE-containing nerves are present near the hilum and extend along bronchial arteries into the lung. Not until birth do AChE-containing nerves appear in intrathoracic structures. These are vagal preganglionic and postganglionic fibers near the trachea, bronchi, and esophagus. AChE-positive ganglion cells are present in the walls of extrapulmonary bronchi at birth, and perimuscular nerve plexuses containing AChE are also present in the bronchial walls. NE-containing nerves are visible in several divisions of the bronchial artery at birth. Three days postnatally, AChE-containing nerves have not yet invaded intrapulmonary structures, but at this stage the adult pattern of adrenergic innervation is present. By the fifth postnatal day, sparse AChE-positive nerves are associated with intrapulmonary bronchi, and rats 9 days old present the adult pattern of cholinesterase-containing nerves.