The ability to sense and respond to changes in oxygen availability is critical for many developmental, physiological, and pathological processes, including angiogenesis, control of blood pressure, and cerebral and myocardial ischemia. Hypoxia-inducible factor-1α (HIF-1α) is a basic-helix–loop–helix (bHLH)containing member of the PER–ARNT–SIM (PAS) family of transcription factors that plays a central role in the response to hypoxia. HIF-1α, and its relatives HIF-2α/endothelial PAS domain protein (EPAS) and HIF-3α, are induced in response to hypoxia and serve to coordinately activate the expression of target genes whose products facilitate cell survival under conditions of oxygen deprivation. When cells are exposed to chronic hypoxia, the protective response can fail, resulting in apoptosis. This study shows that transcription of the gene encoding Nip3, a proapoptotic member of the Bcl-2 family of cell death factors, is strongly induced in response to hypoxia. The Nip3 promoter contains a functional HIF-1-responsive element (HRE) and is potently activated by both hypoxia and forced expression of HIF-1α. Exposure of cultured cells to chronic hypoxia results in the accumulation of a protein recognized by antibodies raised against Nip3. This study demonstrates a direct link between HIF-1α and a proapoptotic member of the Bcl-2 family and offers a reasonable physiological function for members of the Bcl-2 subfamily, including Nip3 and its close relative Nix. These observations indicate that Nip3 may play a dedicated role in the pathological progression of hypoxia-mediated apoptosis, as observed after ischemic injury.