Previous studies from our laboratory showed an atropine-sensitive component in the hyperresponsiveness of ragweed-sensitized canine tracheal smooth muscle (TSM) in vitro to histamine and potassium. The present studies were undertaken to elucidate the nature of the parasympathetic element in this hyperresponsiveness. TSM strips were dissected from ragweed-sensitized and littermate control dogs and their isometric force generation was measured in vitro. Mechanical responses of sensitized TSM were characterized by hyperreactivity (upward shift of the dose-response relationship) to acetylcholine (ACh), atropine-sensitive spontaneous base line activity and prolonged isometric force plateaus. Control TSM did not contract spontaneously and basal tone was maintained passively. However, eserine could produce spontaneous base-line activity and prolonged isometric force plateau in control TSM that mimicked that observed naturally in sensitized TSM. Sensitized TSM was supersensitive (leftward shift of the dose-response relationship) to ACh and electrical field stimulation, and showed a significant leftward shift of the threshold dose to carbamylcholine (carbachol). However, sensitized and control TSMs were equally reactive to carbachol at doses of 10(-8) M and greater. Also, ACh dose-response curves of sensitized and control TSMs in the presence of the cholinesterase inhibitor eserine (10(-8) M) showed no significant differences in sensitivity or reactivity. These results were consistent with a role for local parasympathetic mechanisms such as altered ACh release and/or breakdown in the hyperresponsiveness of ragweed-sensitized canine TSM.