The response of breathing patterns to increased expiratory resistance is not only of physiologic interest, with respect to the control of breathing, but also of clinical interest because of its clinical relevance to obstructive diseases such as asthma and emphysema. To elucidate the response of breathing patterns to increased expiratory resistance during anesthesia, the respiratory effects of expiratory flow-resistive loading on breathing patterns were studied in 15 conscious and 10 lightly anesthetized subjects. Inspiratory time, expiratory time, respiratory frequency, inspiratory duty cycle, tidal volume, minute ventilation, and mean inspiratory flow rate were determined from a respiratory inductive plethysmograph. End-tidal CO2 was continuously recorded. In awake subjects, respiratory frequency was reduced without change in tidal volume or mean inspiratory flow rate, and minute ventilation was significantly decreased; the synchrony between rib cage and abdomen wall motion was well maintained during the loads. In contrast, in anesthetized subjects, respiratory frequency was reduced with remarkable increases in tidal volume, mean inspiratory flow rate, and minute ventilation, whereas coordination between rib cage and abdomen compartments was disturbed. End-tidal CO2 did not change in conscious subjects, but it increased in anesthetized subjects during the loads. These results indicate that there are differences between conscious and anesthetized subjects in breathing patterns during expiratory loading, and suggest that the ability to coordinate rib cage-abdomen wall motion is easily disturbed during anesthesia in patients with expiratory flow limitation.