The reactivity of cat tracheal smooth muscle to histamine in vitro was studied at various degrees of inflammation of the airways induced experimentally by the intratracheal administration of turpentine oil or Staphylococcus aureus, both in aerosol form. Tracheal smooth muscle preparations from the control animals did not respond to histamine in doses of 10-9–10-3 mol l-1. In tracheal preparations from three groups of cats with turpentine oil inflammation induced 24 h, 48 h and 15 days previously, histamine caused contractions in 20, 70 and 24% of the cats, respectively, according to the degree of inflammation. All tracheal preparations from cats with staphylococcal inflammation responded to histamine by contraction. Atropine, acetylosalicylic acid and phentolamine did not abolish histamine contractions in tracheal preparations, but clemastine did.