The effects of i.v. and local anaesthetics on guineapig tracheal smooth muscle have been investigated in vitro. Diazepam, propanidid and ketamine induced relaxation and antagonized histamine-, acetylcholine-and potassium chloride-induced contractures in the tracheal smooth muscle. The relaxing effects of the anaesthetics were not affected by propranolol. Thiopentone, at concentrations which would be used clinically, caused contracture of the tracheal smooth muscle. Morphine induced relaxation and antagonized histamine-induced contracture, but did not antagonize acetylcholine-or potassium chloride-induced contractures. Lignocaine and procaine, at clinical concentrations, induced contracture of tracheal smooth muscle. Large doses of lignocaine relaxed the tracheal muscle, but large doses of procaine induced spontaneous rhythmical movements. Lignocaine and procaine antagonized contractures induced by histamine, acetylcholine and potassium chloride.