HIF (hypoxia-inducible factor) is the main transcription factor activated by low oxygen tensions. HIF-1alpha (and other alpha subunits) is tightly controlled mostly at the protein level, through the concerted action of a class of enzymes called PHDs (prolyl hydroxylases) 1, 2 and 3. Most of the knowledge of HIF derives from studies following hypoxic stress; however, HIF-1alpha stabilization is also found in non-hypoxic conditions through an unknown mechanism. In the present study, we demonstrate that NF-kappaB (nuclear factor kappaB) is a direct modulator of HIF-1alpha expression. The HIF-1alpha promoter is responsive to selective NF-kappaB subunits. siRNA (small interfering RNA) studies for individual NF-kappaB members revealed differential effects on HIF-1alpha mRNA levels, indicating that NF-kappaB can regulate basal HIF-1alpha expression. Finally, when endogenous NF-kappaB is induced by TNFalpha (tumour necrosis factor alpha) treatment, HIF-1alpha levels also change in an NF-kappaB-dependent manner. In conclusion, we find that NF-kappaB can regulate basal TNFalpha and, in certain circumstances, the hypoxia-induced HIF-1alpha.