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The effects of dexamethasone on the actions of histamine on isolated tissue and large bronchus preparations and the interactions of these substances with intramural neural structures were studied. Low histamine concentrations (10−12–10−8 g/ml) decreased muscle responses induced by stimulation of preganglionic nerve fibers, while high concentrations (10−7–10−4 g/ml) increased these responses. Dexamethasone at concentrations of 10−7–10−6 g/ml decreased muscle responses, while concentrations of 10−5–10−6 g/ml produced biphasic changes in responses. Dexamethasone decreased the effects of histamine at high concentrations. Atropine eliminated the effects of simultaneous application of histamine and dexamethasone on respiratory tract preparations; hexamethonium blocked the effects of substances associated with decreased responses and had virtually no effect on those potentiating responses. Novocaine eliminated the actions of histamine at low and high concentrations and the dilatory effect of dexamethasone. These experimental results led to the conclusion that changes in the responses of muscles from the rat respiratory tract induced by stimulation of preganglionic nerve fibers were modified by low concentrations of histamine and dexamethasone and that these modifications were associated with interactions of these substances with tracheobronchial receptors.