We have studied the effects of ketotifen (Ke), an effective antiallergic drug, on guinea pig trachea. Ke (10^-6-5 × 10^-5M) inhibits contractions induced in this preparation by acetylcholine, prostaglandin F₂α barium chloride, potassium chloride and electrical stimulation. These results provide evidence for protective effects of Ke in airways. Furthermore, Ke exerts also a neural inhibitory effect on postganglionic parasympathetic fibers of guinea pig trachea. These findings resemble the effects reported previously for disodium cromoglycate by us and might be relevant for antiasthmatic and antiallergic properties of Ke.