Introduction: Preterm infants have risks of developing vitamin D deficiency. Thus we aimed to investigate the effect of vitamin D on hyperoxia-induced lung injury in newborn rats. Methods: Full term rat pups were included in the study 12-24 hr after delivery. The pups were randomly divided into eight groups as follows: normoxia control group (NC), normoxia plus vitamin Dgroup (ND1, 1 ng/gr/day vitamin D), normoxia plus vitaminDgroup (ND2, 3 ng/gr/day vitamin D), normoxia plus vitamin D group (ND3, 5 ng/gr/day vitamin D), hyperoxia control group (HC), hyperoxia plus vitamin D group (HD1, 1 ng/gr/day vitamin D), hyperoxia plus Vitamin D group (HD2, 3 ng/gr/day vitamin D), hyperoxia plus vitamin D group (HD3, 5 ng/gr/day vitamin D). The histopathological effects of vitamin D were assessed by alveolar surface area (with mean linear intercept (MLI) method), apoptosis index and proliferating cell nuclear antigen (PCNA) index. Results: MLI values were significantly lower among three groups (HD1: 83.93 +/- 1.95 mu m, HD2: 81.76 +/- 1.68 mu m, and HD3: 82.33 +/- 1.87 mu m) when compared with HC group (92.98 +/- 2.09 mu m) (P = 0.001, P = 0.0004, P = 0.002, respectively). Apoptotic cell index were significantly lower among three treatment groups (HD1: 1.455 +/- 0.153, HD2: 0.575 +/- 0.079, and HD3: 0.700 +/- 0.105) when compared with HC group (2.500 +/- 0.263) (P = 0.001, P = 0.001, P = 0.001, respectively). Although PCNA positive cell index did not change in HD1 group (0.132 +/- 0.008) (P> 0.05), there were significant increases in HD2 (0.277 +/- 0.026) and HD3 (0.266 +/- 0.018) group when compared with HC group (0.142 +/- 0.010) (HD2 P = 0.001, HD3 P = 0.001). Conclusion: Vitamin D seems to protect hyperoxia-induced lung injury in newborn rats. (C) 2016 Wiley Periodicals, Inc.