Background/Aims: Genes encoding for some of the mitochondrial proteins are under the control of the transcriptional factor hypoxia inducible factor-1a (HIF-1α), which can accumulate under normoxic conditions in inflammatory states. The aim of this study was to evaluate the effects of cobalt chloride (CoCl2, α hypoxia mimicking agent), tumour necrosis factor-a (TNF-α) and toll-like receptor (TLR) -2, -3 and -4 agonists on HIF-1α accumulation, and further on HIF-1α-mediated modulation of mitochondrial respiration in cultured human hepatocytes. Methods: The human hepatoma cell line HepG2 was used in this study. Cells were treated with CoCl2, TNF-a and TLR-2, -3 and -4 agonists. HIF-1a was determined by Western blotting and mitochondrial respiration in stimulated cells by high-resolution respirometry. Results: CoCl2, TNF-α and TLR agonists induced the expression of HIF-1α in a timedependent fashion. TNF-α and CoCl2, but not TLR agonists, induced a reduction in complex I-, II- and IV-dependent mitochondrial oxygen consumption. TNF-α-associated reduction of cellular oxygen consumption was abolished through inhibition of HIF-1a activity by chetomin (CTM). Pretreatment with cyclosporine A prevented CoCl2-induced reduction of complex I- and II-dependent mitochondrial oxygen consumption and TNF-ainduced reduction of complex-I-dependent respiration, implicating the involvement of the mitochondrial permeability transition pore openings. TNF-α and TLR-2, -3 and -4 agonists induced the expression of vascular endothelial growth factor, which was partially abolished by the blockage of HIF-1α with CTM. Conclusions: The data suggest that HIF-1α modulates mitochondrial respiration during CoCl2 and TNF-α stimulation, whereas it has no effect when induced with TLR-2, -3 and -4 agonists.