Abstract. Objective: To assess the effects of C1 inhibitor (INH) administration and r-SP-C surfactant application on oxygenation and lung histology in an acute respiratory distress syndrome model. Design and setting: Randomized, controlled experimental study in an animal research laboratory. Material: 36 adult male Sprague-Dawley rats. Interventions: Animals were subjected to repetitive lung lavage. Four experimental groups and two control groups were studied: groups 1 and 2 served as controls. Animals of groups 36 received 200U/kg body weight C1-INH (group 3), 25mg/kg r-SP-C surfactant (group 4) or both (group 5) at 60min postlavage (pl). Animals of group 6 were treated with 200U/kg C1-INH1 at 10min pl. Animals of group 1 were killed 60min (min) pl, animals of groups 26 were killed at 210min pl. Thereafter the lungs were excised for histological examination. Measurements and results: Hyaline membrane formation, intra-alveolar neutrophil (PMN) accumulation and intra-alveolar/perivascular haemorrhage were graded semiquantitatively (04). Blood gases were determined 120, 150, 180 and 210min pl. At 210min pl pO2 in group 4 (45674mmHg) and group 5 (387155mmHg) was significantly higher than in controls (7229mmHg) or after C1-INH monotherapy (group 3: 120103, group 6: 6312mmHg). PMN infiltration after C1-INH monotherapy was significantly less severe than in controls. The combination of r-SP-C surfactant and C1-INH led to significantly lower PMN infiltration than surfactant monotherapy. Conclusion: In this lavage-induced acute respiratory distress syndrome model the administration of C1-INH might be followed by a higher clinical efficacy of exogenously supplied recombinant SP-C surfactant.