Surfactant protein D (SP-D), which has structural homology to C-type lectin binding regions, may play a role in host defense and has no known surfactant function. Because other surfactant proteins have been shown to be increased after prolonged periods of hyperoxia, we sought to evaluate the early effects of hyperoxia (95% O2) on expression of SP-D in the adult male rat lung. Animals were exposed to air or to 12, 36, or 60 h of 95% O2. Northern blot analysis of total lung RNA revealed marked SP-D mRNA increases at 12 h 95% O2 compared with air-exposed controls, with decreasing expression to near that of air-exposed animals by 60 h. Semiquantitative in situ RNA hybridization demonstrated parallel results, with increased numbers of labeled alveolar epithelial (AE) and bronchiolar epithelial (BE) cells at 12 h and increased intensity of labeled alveolar cells, compared with air-exposed controls. After 60 h of exposure to 95% O2, mRNA label intensity in AE and BE was decreased to levels near those seen in air-exposed animals. In contrast, Western blotting showed a decline in total lung SP-D with 95% O2 exposure, beginning at 12 h and continuing at 36 and 60 h, respectively. Semiquantitative immunohistochemistry demonstrated a decline in AE labeling parallel to the total lung Western blot results, but labeled total BE cell numbers increased (P = 0.10). Hyperoxia had differential effects on SP-D abundance in AE and BE cells, and therefore may influence the availability of SP-D to bind microbial pathogens in the airways depending on cell type and location.