The effects of a water extract of cotton bract (CBE) on the activity of trachealis smooth muscle of cats, dogs, and guinea-pigs were examined, and the results compared in an endeavor to develop an animal model for byssinosis. CBE caused contractions of dog trachealis muscle (DT) and guinea-pig trachealis muscle (GPT). The CBE induced contractions were not mediated by histamine, 5- hydroxytryptamine (5HT), or muscarinic receptors. The molecular weight of the active agent of CBE was shown to be less than 14,000 daltons by dialysis. Contractions induced by histamine and relaxation responses to isoproterenol were potentiated by CBE in DT, but were unchanged in GPT. Maximum responses to 5HT were increased in DT and decreased in GPT. Responsiveness to potassium-chloride (KCl) was reduced by CBE in GPT, but unchanged in DT. Responsiveness to methacholine and isoproterenol was unchanged in both species. Cholinergically induced contractions were potentiated by CBE in DT, while adrenergic inhibitory responses were unaffected by CBE. In GPT, neither cholinergic excitatory nor adrenergic inhibitory responses were altered by CBE, while inhibitory responses of the nonadrenergic system, not present in DT, were enhanced by CBE. Reactivity of GPT to adenosine-triphosphate and adenosine, proposed transmitters of the nonadrenergic system, was potentiated by CBE. In cat trachealis smooth muscle (CT), maximum response to 5HT was unaltered by CBE. Responsiveness to KCl was potentiated by CBE. There was also a nonsignificant trend suggesting increased responsiveness to isoproterenol. Attempts to study the effects of Escherichia-coli lipopolysaccharide on the responsiveness of CT revealed equivocal results. The authors conclude that there are marked species differences in the acute effects of CBE on airway smooth muscle.