BACKGROUND: Bronchoconstriction is known to be induced by hypocarbia or hypercarbia. But the above effect has not been studied during general anesthesia. This study was proposed to investigate the effects of hypocarbia and hypercarbia on the respiratory system mechanics in 0.5 MAC enflurane anesthetized cats. METHODS: Six cats, weighing 3.0~3.6 kg were used. Pentobarbital sodium was intraperitonially injected to induce anesthesia and endotracheal intubation was followed. The anesthesia was maintained by 0.5 MAC enflurane, oxygen, and air (FiO2; 0.5). Intermittent mandatory ventilation was applied with Siemens Servo 900C ventilator. The inspiratory flow rate and tidal volume were fixed througout the experiment. Only the respiratory rate was adjusted to achieve normocarbia(PaCO2; 31~38 mmHg), hypercarbia(PaCO2; 38~45 mmHg) and hypocarbia(PaCO2; 24~31 mmHg), which were done not in the order. We used the flow-interruption technique to measure respiratory mechanics. The course of changes in the pressure along with the prefixed flow rate and volume were monitored and recorded with Bicore CP100 pulmonary monitor. The data were transfered to a PC and analyzed by Anadat processing software. Total respiratory system, airway and tissue viscoelastic resistances, and dynamic and static compliances were calculated for normocarbia, hypercarbia and hypocarbia. RESULTS: There are no significant differences of resistances and compliances of respiratory system among hypocarbia, normocarbia and hypercarbia. CONCLUSIONS: The changes in PaCO2 do not influence significantly the resistances and compliances measured by the flow interruption technique used in the study.