In seven postoperative patients with normal preoperative pulmonary function tests, we evaluated the oxygen cost of breathing (VO2resp) during continuous positive pressure ventilation (CPAP) and during a 15 cm H2O inspiratory pressure support ventilation (IPSV). For both periods, VO2 resp was estimated as the difference between total oxygen uptake of the period (VO2tot), measured by a mass-spectrometer system, and that during controlled ventilation. During CPAP ventilation, VO2resp was found to be 11.2 +/- 1.4 percent of VO2tot. During IPSV, VO2resp was found insignificant. It is concluded that a 15 cm H2O IPSV takes over the major part of the work of breathing in postoperative patients without preexisting pulmonary disease.