BACKGROUND: Nephrin, a recently identified protein, could be a slit diaphragm component and it has been suggested to play a crucial role in maintaining the glomerular filtration barrier. It has been reported that mutations in the nephrin gene lead to congenital nephrosis. However, the expression of nephrin in acquired glomerular disease has not yet been fully clarified. We induced nephrotic-range proteinuria in experimental animal and performed morphologic analysis with immunoelectron microscopy. This study was designed to examine the expression and distribution of nephrin in acquired glomerular disease and to suggest a role of nephrin in pathogenesis of proteinuria. METHODS: Twenty-three rats were divided into 3 experimental groups and control(n=6). 17 rats of experimental groups had intravenous injection of puromycin aminonucleoside(PAN) singly, and were sacrificed at 1 week(n=5), 2 weeks(n=6) and 3 weeks(n= 6) later. The expression of nephrin was observed by immunoelectron microscopy employing the polyclonal antibody against nephrin and gold particle. For quantifications, the gold particles were counted from photographs. RESULTS: The average length of foot process in 1 week group(2,307+/-524 nm) was far greater than that of control(317+/-45 nm). The average number of total gold particles per unit length(10,000 nm) of the GBM was reduced at 1 week(4.4+/-1.3), compared with control(12.1+/-3.9). Also, the average number of junctional gold particles at 1 week(1.7+/-0.5) was decreased compared with control(6.7+/-2.2). No difference was observed in the number of junctional gold particles per slit diaphragm among groups. But, there were significant differences in the distribution of gold particles among groups. Gold particles were seen more frequently at apical plasma membrane and cytoplasm in 1 week group, whereas those were observed prominently at junctions in control. CONCLUSION: These data show that the expression of nephrin was decreased with effacement of foot process in PAN induced nephrosis rat. However, nephrin was preserved at not-damaged slit diaphragm. And the distribution of nephrin was changed in PAN nephrosis. Further studies for nephrin production and redistribution should be needed to understand pathogenesis of nephrotic syndrome.