BACKGROUND: Volatile anesthetics are potent bronchodilators of airway smooth muscle. Bronchodilation is occured by depressing reflex neural pathways innervating airway smooth muscle and by acting directly on the smooth muscle cell. We studied the direct relaxation effect and their potency of halothane, isoflurane and desflurane on isolated rat distal bronchial segment of fourth order precontracted with acetylcholine. METHODS: Isolated Sprague-Dawley rat bronchial rings were suspended in tissue bath with modified- Krebs's solution. Based on the dose-response curve, the ED50 of acetylcholine (ACh) was calculated for fourth bronchial segment and administered to each tissue bath, after which the stabilized response was recorded. After then each bronchial segment with intact epithelium was exposed to increasing concentration of halothane, isoflurane and desflurane (1.0, 1.5 and 2.0 MAC)and the relaxant responses were recorded by polygraph. RESULTS: Halothane, isoflurane and desflurane produced concentration-dependent bronchodilation (P<0.05 for either anesthetics; 22 4%, 18 6%, 24 5% for halothane, isoflurane and desflurane at 1.5 MAC, 32 6%, 27 7%, 38 5% for halothane, isoflurane and desflurane at 2.0 MAC). Overall, desflurane had a significantly greater relaxing effect as compared with halothane, isoflurane at 1.5 MAC and 2.0 MAC during ACh-mediated contraction. CONCLUSION: The mechanism of halothane, isoflurane and desflurane-mediated bronchodilation is not known, but may be due to an epithelium dependent effect. The potency of relaxing the ACh precontracted isolated rat bronchial smooth muscle is desflurane, halothane, isoflurane in ordered.