Long-term treatment (8 and 13 weeks) of rats with FGF-2 led to albuminuria and to increase in serum creatinine indicating the development of chronic renal failure. Histologically, the classic picture of focal segmental glomerulosclerosis (FSGS) was found; males were more severely affected than females. Among the early changes podocyte lesions were most prominent. Surprisingly, mitotic figures in podocytes and a considerable fraction of bi(multi)nucleated podocyte profiles were found in treated animals (roughly 16% in males, 8% in females). Since an increase of cell number of podocytes was not evident, we conclude that FGF-2 stimulates podocytes to re-enter the cell cycle and to undergo mitosis (nuclear division). However, podocytes-probably due to their highly differentiated cell shape in the adult-are unable to complete cell division (cytokinesis) resulting in bi- or multinucleated cells; in others cell division may fail totally leading to podocyte degeneration. Most podocytes in FGF-2-treated rats exhibited degenerative changes including cell body attenuation, extensive pseudocyst formation, widespread foot process effacement, as well as detachments from the glomerular basement membrane (GBM). The development of FSGS in this model is very uniform. In the case of podocyte detachments from peripheral capillaries, parietal cells become attached to naked GBM-areas, establishing the nidus for development of a tuft adhesion to Bowman's capsule. Tuft adhesions grow by encroaching of parietal cells onto adjacent capillary loops, resulting eventually in a solid synechia with collapsed capillaries, that is, what represents segmental sclerosis. The distribution of adhesions on the inner surface of Bowman's capsule appeared to be random, including all locations between the vascular and urinary pole. The two main aspects of this study (inability of podocytes to replicate and development of FSGS based on progressing podocyte degeneration) may be part of a vicious cycle. FGF-2 stimulates podocytes to enter cell division thereby conveying them into a hazardous situation. If a podocyte fails and degenerates it cannot be replaced, aggravating the situation for the remaining cells and possibly increasing their predisposition to respond to mitogenic stimuli. Similar mechanisms may constitute the development of FSGS in other experimental as well as human glomerulopathies.