Analysis of the fibroblast growth factor-2 (FGF-2 or bFGF) proteins during chicken embryonic pattern formation and organogenesis revealed that three isoforms (18.5, 20.0, and 21.5 kDa) were synthesized by alternative translation initiation from one coding region. A highly specific antiserum was raised and used for studying the temporal and spatial distribution of the FGF-2 isoforms during chicken embryogenesis. The distribution of FGF-2 proteins during limb pattern formation has been unraveled. Their presence in both ectodermal and mesenchymal cells is consistent with an involvement in regulating the balance of growth and differentiation. High levels of FGF-2 proteins were furthermore detected in all epithelial cells of the developing kidney from the pronephric stage onward. The proteins were in general predominantly cytoplasmic, but a specific subpopulation of limb mesenchymal cells and kidney epithelial cells (podocytes) showed a striking nuclear localization. Nuclear translocation of the FGF-2 proteins occurred in differentiating podocytes of meso- and metanephric glomeruli and was maintained in adult kidneys. These results, in contrast to previous in vitro studies, revealed that nuclear accumulation of FGF-2 proteins is restricted to few specific cells during embryogenesis.