Vascular endothelial growth factor (VEGF) plays an important role in early embryonic vasculogenesis. To establish its temporal expression and localization in the heart during development, we studied rat hearts from the first embryonic day (E) of myocardial vascular tube formation through the early postnatal period. Ventricular VEGF immunoreactivity was noted in the epicardium and the thin underlying myocardium in E10 ventricles. During the earliest stages of vascularization (E13-E16) immunoreactivity was highest in the compact myocardium nearest the epicardium, and subsequently (E18 and thereafter) became more evenly distributed transmurally. By birth (E22) immunoreactivity was most intense around microvessels. Similarly, VEGF mRNA localization, demonstrated by in situ hybridization, was initially highest near the epicardium and then became more evenly distributed transmurally by late gestation. Within the interventricular septum, the highest expression occurred in the middle of the wall where it correlated with the greatest vascularization. Northern blot analysis showed that from E12 through the first 10 days of postnatal life, VEGF was two to three times higher than in the adult. Western blot analysis showed that VEGF tended to be higher in the atria than the ventricles, and negligible in the outflow tract. Our data indicate that VEGF localization and expression 1) correspond to the pattern of vascularization in the embryonic/fetal heart, and 2) remain high during the early postnatal period when capillary proliferation is high. Because VEGF is stimulated by hypoxia, its preferential mRNA expression near the epicardium, that is, farthest from the ventricular lumen and the O2 source, fits with the hypothesis that a hypoxic gradient is a driving force in the transmural vascularization process.