In order to investigate the significance of the histological change in glomerular epithelial cells in minimal change nephrotic syndrome in man (MCNS) and to help in clarifing the mechanism of action of a steroid in this disease, methylprednisolone was administered to rats with puromycin aminonucleoside nephrosis (PAN). This is an experimental nephrosis having a close resemblance morphologically and physiologically, to human MCNS. Morphological changes in the glomerulus were observed ultrastructurally. The administration of the steroid to PAN rats showed remarkable changes including, rapid disappearance of proteinuria in PAN rats in a manner similer to that seen in human MCNS, and significantly faster recovery of changes in glomerular epithelial cells when compared with spontaneous recovery. From the present study, it is clear that the steroid is effective in rapidly restoring the normal shape of glomerular epithelial cells in PAN rats. The filtration barrier in the glomerular capillary wall (GCW) is also thought to have recovered and proteinuria is cured. Based on these considerations, it may be suggested that proteinuria in human MCNS is caused by changes in glomerular epithelial cells, and that the clinical treatment of proteinuria in MCNS is effective when glomerular epithelial cells have functionally recovered.