BACKGROUND: The aim of this study was to compare the effects of isoflurane and enflurane on respiratory resistance using flow-interruption technique. METHODS: Twenty one cats were divided into 3 groups according to the agents administered; Control(control), Isoflurane(1 MAC of isoflurane) and Enflurane(1 MAC of enflurane) groups. Tracheal pressure was measured at 2 cm beyond the distal end of the tube. After measuring the baseline value, methacholine chloride(25 microgram/kg/min) was infused to induce bronchoconstriction which was continued throughout the experiment. Anesthetics were administered for each group 15 minutes after methacholine infusion (control value) via low pressure inlet of the ventilator. Measurements were made every 15 minutes. Intermittent mandatory ventilation was applied with Servo 900C ventilator. Inspiratory flow rate and tidal volume were fixed throughout the experiment for each subject. Pressure, volume and flow were monitored with Bicore CP100 pulmonary monitor. The data were transferred to a personal computer and analyzed by a processing software. Respiratory system, airway and tissue resistances, and dynamic and static compliances were calculated. RESULTS: Methacholine infusion increased both airway and tissue resistances. Fifteen minutes after administering inhalation anesthetics(M30), airway resistances for isoflurane and enflurane decreased to 50.8+/-4.7% and 62.5+/-4.9% of the control value(p<0.05). And the values of tissue resistances for isoflurane and enflurane decreased to 54.7+/-6.2% and 68.0+/-4.4% of the control value respectively (p<0.05). There were significant differences between the isoflurane and enflurane in the values of airway and tissue resistances at M30(p<0.05). But there were no significant differences between the two agents in the values of airway and tissue resistances at M45. CONCLUSION: For isoflurane and enflurane, both airway and tissue resistances are reduced. Isoflurane is more potent than enflurane in reversing methacholine-induced bronchoconstriction in this animal model.