The effect of hypoxia on the expression and secretion of major adipokines by human preadipocytes has been examined. Hypoxia (1% O2) led to an increase in the HIF-1a transcription factor subunit in cultured preadipocytes, as did incubation with the hypoxia mimetic CoCl2. Leptin mRNA was essentially undetectable in preadipocytes incubated under normoxia (21% O2), but exposure to 1% O2, or CoCl2, for 4 or 24 h resulted in an induction of leptin gene expression (measured by real-time PCR). Immunoreactive leptin was not detected in the medium from normoxic preadipocytes, but was present in the medium from the hypoxic cells. Hypoxia stimulated expression of the GLUT-1 facilitative glucose transporter gene and the vascular endothelial growth factor (VEGF) gene in preadipocytes, as in adipocytes. PPARg and aP2 mRNA levels, markers of adipocyte differentiation, were reduced by hypoxia in both cell types. In marked contrast to adipocytes, interleukin-6 (IL-6), angiopoietin-like protein 4, and plasminogen activator inhibitor-1 expression by preadipocytes was not stimulated by low O2 tension. Consistent with the gene expression results, VEGF release into the medium from preadipocytes was increased by hypoxia, but there was no change in IL-6 secretion. It is concluded that hypoxia induces human preadipocytes to synthesize and secrete leptin. Preadipocytes and adipocytes differ in their responsiveness to low O2 tension, maturation of the response to hypoxia developing on differentiation.