During early embryogenesis, the pancreas shows a paucity of blood flow, and oxygen tension, the partial pressure of oxygen (pO2), is low. Later, the blood flow increases as β-cell differentiation occurs. We have previously reported that pO2 controls β-cell development in rats. Here, we checked that hypoxia inducible factor 1α (HIF1α) is essential for this control. First, we demonstrated that the effect of pO2 on β-cell differentiation in vitro was independent of epitheliomesenchymal interactions and that neither oxidative nor energetic stress occurred. Second, the effect of pO2 on pancreas development was shown to be conserved among species, since increasing pO2 to 21 vs. 3% also induced β-cell differentiation in mouse (7-fold, P