The total cholinesterase activity in canine tracheal smooth muscle was found to consist of butyrylcholinesterase and acetylcholinesterase in a ratio of 3:1. Most of the acetyl- and butyrylcholinesterase sites were distributed on the muscle surface; the remaining hydrolytic sites were associated with internal structures. Intracellular acetylcholinesterase staining was associated with the perinuclear envelope, sarcoplasmic reticulum and Golgi apparatus. Intracellular butyrylcholinesterase was associated with the perinuclear envelope, sarcoplasmic reticulum and the contractile filaments. Inhibition of acetylcholinesterase by the selective agent 1,5,bis(allyl-dimethylammoniumphenyl)-pentane-3-one dibromide (BW 284C51) led to a parallel leftward shift in the concentration-response curve for bath-applied acetylcholine. A similar shift was observed in the frequency-response curve for neurally released acetylcholine. Inhibition of butyrylcholinesterase by the selective agent tetraisopropyl-pyrophosphoramide potentiated the response to bath-applied and neurally released acetylcholine; the potentiation was limited to acetylcholine concentrations greater than or equal to 1 microM and frequencies greater than or equal to 10 Hz. It is concluded that both acetyl- and butyrylcholinesterase participate in the hydrolysis of acetylcholine in canine tracheal smooth muscle. The role of acetylcholinesterase is evident over the entire range of concentrations (1 nM to 100 microM) and frequencies (1 to 90 Hz) examined, whereas the role of butyrylcholinesterase is confined to the higher end of the concentration and frequency ranges used.