BACKGROUND: Nitric oxide (NO) is a selective pulmonary vasodilator, and inhaled NO has bronchodilatory action due to their relaxation effect on conducting airway smooth muscle. The aim of this study was to evaluate the effects of inhaled NO on respiratory system mechanics in cats. METHODS: Nineteen cats were divided into 3 groups according to the doses of NO administered; group C (control, n=7), group 20 (20 ppm of NO, n=7), and group 40 (40 ppm of NO, n=5). After measuring the baseline value, methacholine chloride 25 microgram/kg/min was infused to induce bronchoconstriction. Inhalation of NO was started for each group 15 minutes after methacholine infusion. Pressure, volume, and flow rate were monitored with Bicore CP100 pulmonary monitor and the data were transferred to a personal computer and analyzed by a processing software. Respiratory system, airway and tissue viscoelastic resistances, and dynamic and static compliances were calculated. RESULTS: Methacholine infusion increased both airway and tissue resistances. Fifteen minutes after inhaling NO, airway resistances for NO 20 ppm and 40 ppm decreased to 65.8+/-8.5% and 62.2+/-8.9% of the control value (p<0.05). The values of tissue resistances for NO 20 ppm and 40 ppm decreased to 72.4+/-10.8% and 78.2+/-10.5% of the control value respectively (p<0.05). And thirty minutes after inhaling NO, there were also decreases of airway and tissue viscoelastic resistances in both groups but had no differences compared with fifteen minutes' values. There were no significant differences between the NO 20 ppm and 40 ppm in the values of airway and tissue viscoelastic resistances. CONCLUSION: Inhaled NO of 20 ppm and 40 ppm decreased both airway and tissue viscoelastic resistances and airway resistance was decreased more markedly than tissue resistance. There were no significant differences between 20 ppm and 40 ppm of NO in respiratory system mechanics in cats.