Using isolated guinea pig trachea that was relatively insensitive to 5-HT, it was shown that, in the absence of 5-HT induced mechanisms, the response to cotton dust aqueous extract (CDE) was only partially (75%) blocked by the antihistamine, chlorpheniramine at concentrations that completely blocked histamine. Methysergide also blocked the CDE-induced response (90%). Given that this preparation is insensitive to 5-HT, the mechanism of action of methysergide is unclear. Other studies have identified actions of methysergide unrelated to 5-HT, including interference with the action of eicosanoids. When the trachea was superfused, dose-dependent responses could be obtained with carbachol but not with CDE.