The innervation of the primary bronchus of the chicken was studied with in vitro pharmacological techniques and with the electron microscope. The primary response of the smooth muscle to field stimulation is relaxation of the muscle and this is not blocked by adrenergic blocking agents. Excitatory cholinergic innervation can be demonstrated when the muscle is partially relaxed. Examination of the ultrastructure of the muscle and nerves shows numerous axon profiles filled with large granular vesicles of the type associated with nonadrenergic or purinergic neurotransmission. Agranular vesicles characteristic of cholinergic innervation are also seen but there is no evidence of adrenergic innervation to the smooth muscle. The smooth muscle cells show connections of the nexus type. These findings indicate that the primary bronchus of the chicken has a dominant inhibitory system and this is nonadrenergic in type.