Vascular endothelial growth factor (VEGF) induces endothelial cell growthin vitro and angiogenesis in vivo. VEGF is a secreted peptide which acts on vascular endothelial cells through two different high-affinity receptors, KDF/Flk-1 and Flt-1, whose mRNA have been identified in developing human lung. VEGF mRNA and protein have also been localized to airway epithelial cells of midtrimester human fetal lung suggesting that capillary development in the lung may be modulated through secretion of VEGF by epithelial cells during development. Midtrimester human fetal lung maintained in explant culture is a well characterized model in which undifferentiated prealveolar epithelial cells differentiate into type II cells after 4 days of incubation in serum-free medium. The aim of the present study was to determine the effects of hypoxia, a cyclic AMP analog and glucocorticoids on VEGF mRNA expression and to determine the pattern of VEGF protein localization in human fetal lung in vitro. Lung tissue explants were incubated in serum free medium in 20% or 2% O2 for 4 days. Additional tissues cultured for 4 days in 20% O2 were further incubated 48 h in 2% O2 and the absence or presence of regulatory factors. Steady state levels of VEGF mRNA were increased in incubated tissues compared to start tissues with the greatest levels in tissues incubated for 4 days in 2% O2 vs 20% O2. Type II cell-containing tissues further incubated in the presence of Bt2cAMP (1 mM) and 2% O2 demonstrated an increase in VEGF mRNA compared to control tissues incubated in 2% O2, whereas, dexamethasone (10-7M) downregulated steady state mRNA levels in 2% O2. Immunostaining for VEGF was weak in the type II cell-deficient starting tissues but markedly increased after 4 days of incubation in both 2% and 20% O2 treated tissues. Staining was limited to the airway epithelium (type II cells in incubated tissues) and a subepithelial zone suggesting production and basalar secretion by type II cells. These data demonstrate the association of VEGF and type II cells in a model of developing human lung. Additionally, changes in steady state VEGF mRNA levels with oxygen, cAMP analog and glucocorticoids suggest that VEGF and perhaps capillary development are regulated by these factors in the developing human lung.