Growth of trophoblast tissue in early pregnancy is rapid and accomplished in an unusually hypoxic environment. Hypoxia has been reported to upregulate mRNA production of vascular endothelial growth factor (VEGF), and VEGF receptors have been found on trophoblast cells. These observations suggest that VEGF may have an important role in early placentation. This study examines the influence of hypoxia on both the production of the VEGF message and protein and on the production of human chorionic gonadotrophin (hCG) protein by the cell lines JEG, JAr and BeWo. Cells were grown under normoxic and hypoxic conditions for 72 h. The average oxygen tension in the culture media of the hypoxic cultures (6-7 kPa) was significantly less than in the normoxic cultures (19-21 kPa). RNA was extracted and message for VEGF(121), VEGF(165) and VEGF(189) found in all cell lines by reverse transcription and the polymerase chain reaction (RT-PCR). These messages were upregulated by hypoxia; findings confirmed by competitive PCR for VEGF and expression of the house keeping gene GAPDH. hCG and VEGF were measured by immunoassay. Hypoxia resulted in an increase in VEGF production (P<0.05) but had inconsistent effects on hCG production. In some experiments the absolute concentrations of hCG and VEGF in the culture media were noted to be significantly correlated (r>0.5, P<0.05). In addition to its role in angiogenesis, VEGF may have direct effects on trophoblast cells encouraging proliferation and invasion. These effects may be regulated in part through oxygen supply and hCG.