Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic helix-loop-helix transcription factor that regulates genes whose products play key roles in maintaining O₂homeostasis. We have previously demonstrated that HIF-1 mRNA, protein, and DNA-binding activity are induced when mammalian tissue culture cells are subjected to hypoxia. In this paper, we report our analysis of HIF-1 mRNA expressionin vivo.We demonstrate expression of HIF-1α and HIF-1β (ARNT) mRNA in all human, rat, and mouse organs assayed and show for the first time that HIF-1 mRNA expression was induced in brain, kidney, and lung when rats or mice were exposed to reduced ambient O₂concentrations for 30 to 60 min. The ubiquitousin vivoexpression of HIF-1α and HIF-1β (ARNT) mRNA is consistent with the proposed role of HIF-1 in coordinating adaptive transcriptional responses to hypoxia.