Vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) stimulate endothelial cell proliferation, migration, and vascular tube formation. We tested the hypotheses that these growth factors stimulate (1) cell migration and (2) assembly into cord-like structures in embryonic rat heart explants cultured on collagen gels. Atrial and ventricular explants from rat embryos at 12 (E12, avascular) and 14 (E14, early vascularization stage) days of gestation were cultured on a collagen substrate. Western blot analysis of the explants indicated that endogenous VEGF was present in both atria and ventricles during incubation. Addition of bFGF to E12 explants markedly increased cell migration, whereas VEGF had no significant effect. In E14 explants neither growth factor influenced cell migration. Cotreatment with VEGF and bFGF did not have a synergistic effect on the migration distance of cells from either E12 or E14 embryonic hearts. However, VEGF stimulated the appearance of cord-like structures in E14, but not E12, explants. Transmission electron microscopy analysis showed that these cord-like structures consist of elongated cells, some of which aggregate into clusters, or form tube-like structures, similar to capillaries. Serial sections of monolayers revealed that tube formation occurs beneath the surface of collagen gel. We conclude that in this model system VEGF and bFGF play distinct roles, at specific time points, in coronary vascular tube formation in the developing heart. ©1995 Wiley-Liss, Inc.