During kidney organogenesis the development of renal vessels must be synchronized with the maturation of nephrons and the collecting duct system. Several reports showed that hormones and mitogenic peptides as basic fibroblast growth factor (bFGF) or vascular endothelial growth factor (VEGF) are involved in this regulatory process. It is a known fact that bFGF receptors are expressed by differentiating tubular epithelium and mesenchyme, but little information is available about the function of bFGF in kidney organogenesis. The role of bFGF during kidney development was investigated using an organotypic culture system and immunohistological techniques. Renal cortex explants were prepared from the kidneys of neonatal rabbits with a microsurgical method, retaining the natural tissue composition. The explants were cultured serum free under continuous medium perfusion. Our results indicate a new and unexpected role of bFGF during the differentiation process. When bFGF alone was applied, vessels could no longer be detected. The inhibitory influence of bFGF could be overcome by addition of VEGF or hormones such as retinoic acid and aldosterone/vitamin D3. The combination of these factors with bFGF resulted in the expression of small vessel-like structures. We conclude that bFGF has a morphogenic rather than a mitogenic function during kidney vessel development.