Injury and repair of the glomerular epithelial cells (GECs) play an important role in the pathogenesis of focal segmental glomerulosclerosis (FSGS). To obtain a better understanding of proliferation and apoptosis of GECs, we examined immunohistochemical and in situ hybridization findings in puromycin aminonucleoside nephrosis (PAN) of rats. The minimal-change nephrotic syndrome model (PAN-MCNS) was induced by administering 5 subcutaneous injections of puromycin aminonucleoside (PA; each 1.5 mg/100 g B/W to one group of rats), whereas the FSGS model (PAN-FSGS) was induced by administering an additional 5 injections of PA to another group of rats. The cell kinetics of the GECs were assessed with labeling 5-bromo 2'-deoxyuridine (BrdU) and proliferating cell nuclear antigen (PCNA). To investigate regulation of apoptosis in rats with PAN, we evaluated the expression of p53, Fas antigen, Fas ligand and Bc1-2. Rats with PAN-MCNS exhibited a significantly greater number of BrdU- and PCNA-labeled GECs as compared with control rats. In rats with PAN-FSGS, the number of PCNA-labeled GECs was greater than in rats with PAN-MCNS, but the number of BrdU-labeled GECs was lower. Apoptotic cells were occasionally observed in the sclerotic lesions, with the number being significantly higher in rats with PAN-FSGS than in rats with PAN-MCNS and control. Apoptotic cells were observed in the GECs of PAN-FSGS rats. However, they were negative for p53, Fas antigen, and Fas ligand. Immunohistochemical and in situ hybridization studies revealed a greater intraglomerular overexpression of Bc1-2 protein and bc1-2 mRNA in the PAN-FSGS rats as compared with control rats. These results suggest that insufficient proliferation and apoptosis in GECs may be involved in the progression of FSGS.