Adipose tissue becomes hypoxic in obesity, and cell culture studies have demonstrated that hypoxia leads to major changes in adipocyte function. Studies on the response of adipocytes to low O₂ tension have employed marked hypoxia (1% O₂). Here, we have examined the effects of modest hypoxia, utilising differing concentrations of O₂ (1-21%), on adipokine production and glucose uptake by human adipocytes. Incubation with 10% O₂ (24 h) increased expression of the leptin, vascular endothelial growth factor (VEGF) and Angptl4 genes, while leptin expression was elevated even at 15% O₂ (compared to 'normoxia'-21% O₂). Overall, there was a concentration-dependent increase in the expression of these genes as O₂ fell, with the highest mRNA level evident at 1% O₂. Parallel changes were observed in the secretion of leptin, VEGF and IL-6 into the medium, an increased release being evident at 10% O₂ (15% O(2) for leptin). Adiponectin gene expression was reduced at 15% O₂ and below, while adiponectin release was significantly reduced at 5% O₂. Both 2-deoxy-D: -glucose uptake and lactate release showed progressive increases as O₂ concentration fell, being significantly raised at 10% and 5% O₂, respectively. The alterations in substrate transport were accompanied by parallel changes in transporter gene expression, GLUT1 and MCT1 mRNA level increasing from 15% and 10% O₂, respectively. These results indicate that marked responses to reduced O₂ concentration are exhibited by human adipocytes at O₂ levels well above those associated with hypoxia and employed in cell culture studies. Adipocytes are sensitive to small changes in O₂ tension.