We studied the changes in end-tidal CO(2) (ET(CO)(2)) and systemic responses after tourniquet deflation in spontaneously breathing and ventilation-controlled patients during orthopedic surgery of both the upper and/or the lower extremities. In most patients, increases in ET(CO)(2), heart rate, and Pa(CO)(2), as well as decreases in blood pressure and pH were observed. In every spontaneously breathing patient, the respiratory rate began to increase before the ET(CO)(2) reached a maximum. Arterial blood gas analysis suggested that the increase in ET(CO)(2) closely reflected the increase in Pa(CO)(2). Our study yielded new information on the ET(CO)(2) changes as follows: 1) the time for ET(CO)(2) level to reach a peak (peak time) was almost constant despite the considerable differences in the increases in ET(CO)(2) both in spontaneous breathing and ventilation-controlled groups and the peak time in the former group was shorter than that in the latter group; and 2) it was suggested that the increase in ET(CO)(2) in the spontaneously breathing patients was smaller than that in ventilation-controlled patients when both patients were subjected to the same conditions on tourniquet time and tourniqueted area. Our data showed that the increase in ET(CO)(2) (or Pa(CO)(2)) can be large and prolonged in some situations. Thus, we recommend continuous ET(CO)(2) monitoring and the proper hyperventilation at tourniquet deflation in order to minimize any adverse effects of acidosis.