Fluctuating CPAP(F-CPAP) is a combination of spontaneous ventilation and fluctuating PEEP, in which end-expiratory pressure (EEP) is periodically changed within a certain range. In a dog model with localized lung injury induced by the aspiration of non-heparinized blood (2 ml.kg(-1) body weight), we carried out a comparative study of the effects of F-CPAP in which the EEP was cyclically changed from 4 to 12 cmH(2)O with periods of 10 min and those of conventional CPAP with a fixed EEP of 8 cmH(2)O (C-CPAP), on hemodynamics and pulmonary oxygenation. The blood aspiration produced significant increases in the intrapulmonary shunt (Qsp/Qt), the alveolar-arterial difference of partial pressure of oxygen (A-aD o(2)), and the respiratory rate (RR). Although both F-CPAP and C-CPAP reduced Qsp/Qt and A-aD o(2) and RR, 7 dogs treated with F-CPAP showed a significantly greater recovery of Qsp/Qt and A-aD o(2) than 7 dogs treated with C-CPAP. There were no significant differences in hemodynamic variables between the two groups. These results suggest that F-CPAP is more useful in the treatment of some kinds of hypoxic respiratory failure due to uneven distribution of lung injury.