The use of a ventilation strategy with high positive end-expiratory pressure (PEEP) that is intended to recruit collapsed alveoli and to prevent recurrent collapse can reduce alveolar protein influx in experimental acute lung injury (ALI). This could affect the pulmonary response to treatment with surfactant, since plasma proteins inhibit surfactant function. We studied the effect of exogenous surfactant on lung mechanics after 4 h of mechanical ventilation with high or low PEEP. Twenty-two adult male Sprague-Dawley rats were anaesthetized, tracheotomized and submitted to pressure-controlled mechanical ventilation with 100% oxygen. One group served as healthy controls (n = 6). In the remaining animals acute lung injury was induced by repeated lung lavages to obtain a PaO2 < 13 kPa during ventilation with a peak inspiratory pressure (PIP) of 26 cm H2O and a PEEP of 6 cm H2O. These animals were allocated randomly to ventilation with high PEEP (n = 8; 100 breaths min-1, I:E = 1:1 PIP 35 cm H2O, PEEP 18 cm H2O) or to conventional mechanical ventilation (PIP 28 cm H2O, PEEP 8 cm H2O; n = 8; ventilated control group). After 4 h of ventilation, all animals were given surfactant (120 mg kg-1) via the trachea and ventilation was continued for 15 min. At the end of the study, pressure-volume curves were constructed to measure total lung capacity at 35 cm H2O (TLC35) and maximal compliance (Cmax), and bronchoalveolar lavage was then used to measure alveolar protein influx. After lavage, PaO2, remained around 13 kPa in the ventilated control group and was > 66 kPa in the high-PEEP group. After surfactant treatment, PaO2 increased to > 53 kPa in both groups. In the ventilated control group alveolar protein influx was greater and TLC35 and Cmax were lower than in the high-PEEP group. We conclude that the pulmonary response to exogenous surfactant after mechanical ventilation in experimental ALI is improved when a ventilation strategy with high PEEP is used.