Postnatal lung function depends on a tightly controlled and complex developmental programme which regulates lung branching morphogenesis, vasculogenesis and fluid secretion. A number of these processes begin during the pseudoglandular stage (weeks 9 – 17 in human, and embryonic day (E)11.5 – 16.5 in the mouse) and occur in a relatively hypercalcemic environment (~1.7 mM v ~1.2 mM for a normocalcemic adult [1]). Previously, our group has demonstrated that this increase in extracellular calcium ([Ca2+o]) is an important extrinsic factor for lung development, suppressing branching and cellular proliferation and enhancing Cl–dependent fluid secretion through activation of the calcium-sensing receptor (CaSR) [2]. To further understand the role of Ca2+o in lung development, we have now examined the effect of fetal hypercalcemic conditions on vasculogenesis using the Flk-1+/LacZ reporter mouse model. Fetal liver kinase (Flk)-1, a receptor for vascular endothelial growth factor-A, is predominately expressed in the early stages of mouse lung development (E9.5 – E13.5), during which vascular endothelial cells have been shown to actively proliferate [3]. E12.5 mouse lung explants cultured for 48 h in medium containing adult Ca2+ concentrations (1.05 mM) showed Flk-1 expression confined to the mesenchymal spaces between developing branches. However, culturing lung explants in the presence of fetal (1.70 mM) [Ca2+o] induced a marked increase in Flk-1 expression, accompanied by a change in localization with the vasculature criss-crossing over and around the developing branches. The addition of the CaSR activator NPS-568 (30 nM) at 1.05 mM Ca2+o did not affect Flk-1 expression, suggesting that the CaSR is not directly involved in regulation of vasculogenesis. Immunohistochemistry of E12.5 whole mouse embryos and human fetal lungs showed expression of a number of voltage-gated calcium channels – including the L- and R-type calcium channels Cav1.2, Cav1.3 and Cav2.3 – within the epithelium of the developing lung, suggesting an alternate mechanism for [Ca2+o] sensitivity. Functional experiments with the L-type calcium channel blocker nifedipine and the R-type calcium channel blocker SNX-482 at 1.70 mM Ca2+o led to significant increases in lung branching (39.8 ± 4.1% n=11 vs. 70.6 ± 4.5% n=9 p<0.01 and 81.1 ± 11.7 n=6 p<0.001 respectively. Values are mean ± SEM, compared by ANOVA) and further increases in Flk-1 expression over 48h. In summary, functional L-/R-type calcium channels may provide an alternative and complementary mechanism for the [Ca2+o] sensitivity of fetal lung development, including branching morphogenesis and vasculogenesis.