Pulmonary surfactant protein-A (SP-A) has been reported to regulate the uptake and secretion of surfactant by alveolar type II cells, to stabilize large surfactant aggregates including tubular myelin, and to protect the surface activity of surfactant from protein inhibitors. In this study we investigated the consequences of overexpression of SP-A on pulmonary homeostasis and surfactant function in transgenic mice. The human SP-C promoter was used to direct synthesis of rat surfactant protein A (rSP-A) in alveolar type II cells and nonciliated bronchiolar cells of the distal respiratory epithelium. Levels of SP-A measured through enzyme-linked immunosorbent assay were 7- to 8-fold higher in lung homogenates and alveolar lavage fluid of the rSP-A mice than in those of transgene-negative littermates. The swimming exercise tolerance and lung compliance of mice bearing the transgene were unchanged. Mean air space sizes seen in randomly selected light-microscopic fields were not significantly different in the transgene-positive and -negative mice by morphometric analysis, but 15% of transgenic animals had scattered foci containing dilated alveoli and alveolar ducts without evidence of inflammation or fibrosis. Some alveolar macrophages contained bar-shaped osmophilic inclusions that had a highly ordered ultrastructure. There were no differences between the transgene-positive and -negative mice in the tissue or alveolar pool sizes of saturated phosphatidylcholine or in the large-aggregate composition of alveolar surfactant. The surface activity of surfactant isolated from the rSP-A mice was similar to that of the controls, but in the presence of protein inhibitors, the surface tension-reducing properties of the rSP-A surfactant were better preserved (P < 0.05). We conclude that overexpression of SP-A does not affect resting surfactant phospholipid levels, but that it enhances the resistance of surfactant to protein inhibition.