Lung HO-1 is developmentally regulated with highest expression in the first days of life. In addition, neonatal mice have limited HO-1 induction in hyperoxia compared to adults. However, few reports have addressed the functional impact of miRNAs in the regulation of HO-1 in vivo. The aims of the present study were to characterize changes in lung miRNA expression during postnatal development and in response to hyperoxic exposure and to identify miRNAs that target lung HO-1 gene expression. Neonatal (<12 h old) or adult mice (2 months old) were exposed to room air or hyperoxia (95% oxygen) for 72 h. TaqMan low density array rodent miRNA assays were used to calculate miRNA expression changes between control and hyperoxia groups in neonatal and adult lungs. In neonates, we identified miR-196a, which binds to the 3'-UTR of Bach1and regulates its expression and subsequently leads to higher levels of lung HO-1 mRNA compared to adults. Despite the increase at baseline, miR-196a was degraded in hyperoxia resulting in limited HO-1 induction in the neonatal mice lungs. Furthermore, the developmental differences in lung HO-1 gene expression can be explained in part by the variation in miRNA-196a and its impact on Bach1. This report is the first to show developmental differences in lung miR-196a and its impact on Bach1 and HO-1 expression at baseline and in hyperoxia.