The effects of 5‐hydroxytryptamine, histamine and acetylcholine on the reactivity of the lung of the anaesthetized dog.

1. Anaesthetized dogs were given aerosols of 5‐hydroxytryptamine (5‐HT), histamine and acetylcholine; the effects on the changes in total lung resistance (RL) and dynamic lung compliance (Cdyn) produced by electrical stimulation of the peripheral cervical vagi, and I.V. histamine and acetylcholine were studied. 2. Pretreatment with 5‐HT significantly potentiated the increases in RL caused by vagal stimulation, histamine or acetylcholine. Pretreatment with histamine significantly potentiated the increases in RL caused by vagal stimulation but the potentiation of the increases in RL produced by histamine and acetylcholine were not significant. Pretreatment with acetylcholine did not potentiate the increases in RL due to vagal stimulation, histamine or acetylcholine. The falls in Cdyn produced by vagal stimulation, histamine or acetylcholine were not potentiated by pretreatment with aerosols of 5‐HT, histamine or acetylcholine. 3. The effects of I.V. histamine, 5‐HT and acetylcholine on RL and Cdyn were studied when given alone and when combined with vagal stimulation. 4. Vagal stimulation significantly potentiated the increases in RL due to 5‐HT and histamine, but did not affect the increases in RL caused by acetylcholine or the falls in Cdyn produced by any of the three agonists. 5. By comparing these results with those from a previous study, it is concluded that, for the three agents studied, in order for increased irritant receptor discharge to reflexly increase RL, the agent has to increase the reactivity of the airway to vagal stimulation. It is also concluded that the degree of vagally mediated broncho‐constriction can be varied by changing either the afferent limb through a change in irritant receptor discharge, or the efferent limb, by a change in airway reactivity.