Basic and acidic fibroblast growth factor (FGF) are two closely related growth factors that interact with the same receptor. Structurally related to FGF are five different oncogenes that have been involved in mammary, stomach, and bladder cancers but whose precise functions in physiologic processes are presently unknown. Both basic and acidic FGF exert similar biologic effects on a wide range of mesoderm- and neuroectoderm-derived cells, acting both as morphogens and mitogens. In addition, basic FGF has been shown to act as a ventrovegetalizing factor, inducing, at the midblastulae stage, mesoderm formation from animal pole cells destined to form ectodermal structures. This primordial role explains the wide variety of tissues, ranging from the central and peripheral nervous system to the vascular system, that are affected by FGF during the late embryonic, neonatal, and adult phases of development.