Continuous hydroxylation of the HIF-1 transcription factor alpha subunit by oxygen and 2-oxoglutarate-dependent dioxygenases promotes decay of this protein and thus prevents the transcriptional activation of many genes involved in energy metabolism, angiogenesis, cell survival, and matrix modification. Hypoxia blocks HIF-1alpha hydroxylation and thus activates HIF-1alpha-mediated gene expression. Several nonhypoxic stimuli can also activate HIF-1, although the mechanisms involved are not well known. Here we show that the glucose metabolites pyruvate and oxaloacetate inactivate HIF-1alpha decay in a manner selectively reversible by ascorbate, cysteine, histidine, and ferrous iron but not by 2-oxoglutarate or oxygen. Pyruvate and oxaloacetate bind to the 2-oxoglutarate site of HIF-1alpha prolyl hydroxylases, but their effects on HIF-1 are not mimicked by other Krebs cycle intermediates, including succinate and fumarate. We show that inactivation of HIF-1 hydroxylation by glucose-derived 2-oxoacids underlies the prominent basal HIF-1 activity commonly seen in many highly glycolytic cancer cells. Since HIF-1 itself promotes glycolytic metabolism, enhancement of HIF-1 by glucose metabolites may constitute a novel feed-forward signaling mechanism involved in malignant progression.