To investigate the in vivo angiogenic activity of placenta growth factor (PIGF) and its heterodimers with vascular endothelial growth factor (VEGF), the induction of neovascularization of these factors in the mouse cornea was studied. VEGF165 is sufficiently potent to stimulate new capillary growth from the limbal vessels. PIGF129/VEGF165 heterodimers also induce corneal neovascularization with a maximal vessel length similar to VEGF165, but with a marked decrease of vessel density. In contrast, PIGF129 has little or no effect in this in vivo angiogenesis assay. The expression of VEGF mRNA and protein is drastically up-regulated by hypoxia in choriocarcinoma cells, whereas expression of PIGF is not affected by the low concentration of oxygen. Up-regulation of VEGF production results in increased formation of PIGF/VEGF heterodimers in these tumor cells. Thus, hypoxia indirectly up-regulates expression levels of PIGF/VEGF heterodimers and modulates VEGF activity when these factors are co-expressed.