The effect of various smooth muscle stimulants and relaxants was examined on isolated rabbit trachea and bronchus. Trachea contracted maximally in response to carbachol, slightly to KCl, and there was no response to serotonin or PGF2alpha. Relaxation of carbachol-contracted trachea was elicited by papaverine, aminophylline, isoproterenol, bradykinin, or PGE2. Histamine also relaxed the rabbit trachea. However, bronchi from the same animal contracted to this amine. As was the case with trachea, the bronchi contracted maximally to carbachol and slightly to KCl; serotonin and PGF2alpha were inactive. Unlike trachea, only papaverine or aminophylline completely relaxedthe rabbit bronchus. The other relaxants tested produced smaller responses.Contractions induced by carbachol were blocked by atropine. Bronchial contractions caused by histamine were antagonized by pyrilamine. In contrast, relaxation of trachea caused by histamine was neither affected by pyrilamine nor burimamide, metiamide, or propranolol. We conclude that a lack of pharmacological uniformity exists in at least two smooth muscle subdivisions of a mammalian airway and that this must be taken into consideration when determining the action of drugs on the respiratory system.