In cats, anesthetized with chloralose, vagotomized and relaxed by infusion of sexamethonium, bronchoconstrictor reactions were induced by the i.v. injection of arachidonic acid (50 to 200 mug/kg). These reactions could be completely suppressed by the following drugs: indomethacin (ED50 0.16 mg/kg), flufenamic acid (1.8 mg/kg), acetylsalicylic acid (2.4 mg/kg) and phenylbutazone (4.7 mg/kg), but paracetamol was only partly effective even in doses up to 50 mg/kg. The results show that rather small doses of most weak analgesics can effectively inhibit prostaglandin biosynthesis in the bronchi and make it likely that this inhibition is involved in the drug-induced bronchoconstrictor reactions, which have been observed clinically.