The transcriptional machinery involved in the transition of an infant from intrauterine to air-breathing life is developmentally regulated, as the fetus and adult manifest differential genetic expression. The low oxygen (O(2)) environment of the mammalian fetus and the increase in O(2) tension that occurs at birth may account for the developmentally regulated alterations in gene expression. We tested the hypothesis that hypoxia-inducible factor 1 (HIF-1) expression, an O(2)-sensitive transcription factor, is developmentally regulated. We found that in fetal pulmonary artery (PA) smooth muscle cells (SMC), fetal HIF-1 protein levels were O(2)-insensitive, whereas in adult PA SMC, hypoxia increased HIF-1 protein expression. Surprisingly, hypoxia increased HIF-1 mRNA expression in fetal, but not in adult, PA SMC. HIF-1 degradation and transcriptional activity is contingent on prolyl- and asparagyl-hydroxylases. To determine whether developmental differences in O(2) sensitivity or expression of these enzymes accounts for the divergence of HIF-1 sensitivity between fetus and adult, we studied the expression of the three most well characterized prolyl-hydroxylases, PHD1, PHD2, and PHD3, and the expression of regulators of HIF-1 transcriptional activity, asparagyl-hydroxylase, factor inhibiting HIF, and the oncogenic factor, CITED2 (CREB-binding protein/p300 interacting transactivator with ED-rich tail). We found that, as in the case of HIF-1, these genes are differentially regulated in the fetus, enabling the mammalian fetus to thrive in the low O(2) tension intrauterine environment even while rendering a newborn infant uniquely well adapted to respond to the acute increase in O(2) tension that occurs at birth.