The response of canine tracheal and bronchial smooth muscle to i.a. administered agonists causing smooth muscle contraction was compared in 22 mongrel dogs in vivo. Tracheal and bronchial contractile responses were measured isometrically in situ in the same dogs. In nine dogs, dose-response curves were generated with i.a. acetylcholine and histamine (10(-10) to 10(-6) mol) in a 4-cm tracheal segment and a 1-cm segment of third order bronchus. The tracheal response to i.a. histamine was 36.5 +/- 4.48% of the response to equivalent doses of acetylcholine. In bronchus, the contraction caused by histamine was 81.0 +/- 2.83% of the cholinergic contractile response (P less than .001). In five dogs having beta adrenergic blockade with propranolol, tracheal contraction to 10(-8) to 10(-6) mol i.a. norepinephrine was 27.3 +/- 4.2% of the response to acetylcholine. However, in bronchus, contraction to norepinephrine was 218 +/- 16.6% of the response to equivalent doses of acetylcholine (P less than .001). Phentolamine (200-400 micrograms/kg i.a.) caused 79 to 100% blockade of the tracheal and bronchial response to i.a. norepinephrine. Cholinergic contraction was blocked specifically with 5 micrograms/kg i.a. of atropine. It is concluded that there is substantial heterogeneity in the contractile responses of canine trachea and bronchus in situ. Relative to cholinergic contraction, both histamine and alpha adrenergic stimulation cause substantially greater contraction of airway smooth muscle in the third order bronchus than in trachea.