Primary antiserum to choline acetyltransferase (ChAT), a specific marker for cholinergic nerves, was used to characterize the distribution of cholinergic nerve fibers and nerve cell bodies in guinea pig airways. ChAT immunoreactive nerve fibers were localized to the smooth muscle throughout the conducting airways and in the lamina propria of the trachea and large bronchi. Likewise, all nerve cell bodies in the ganglia intrinsic to the trachea and bronchi displayed a cholinergic phenotype. By contrast, ChAT immunoreactive nerve fibers were infrequently seen in the lamina propria of the peripheral airways and were absent in the airway epithelium. No evidence for colocalization of ChAT and the enzyme synthesizing the putative relaxant neurotransmitter nitric oxide was observed. These results provide further evidence for the key role played by cholinergic nerves in regulating airway smooth muscle tone and bronchial blood flow and provide further evidence that acetylcholine is not coreleased with the neurotransmitter(s) mediating relaxations of airway smooth muscle.