The mechanism of selective albuminuria in minimal change nephrotic syndrome, in which glomerular capillaries are diffusely covered by effaced podocyte foot processes with reduced slit diaphragms, is unknown. Podocyte injury is due, in part, to NADPH-induced oxidative stress. Here we studied mechanism of selective albuminuria in puromycin aminonucleoside (PAN) nephrotic rats, a model of minimal change nephrotic syndrome. In these rats, Evans Blue-labeled human albumin was taken up by podocytes and its urinary excretion markedly increased, with retained selectivity for albumin. Immunogold scanning electron micrographic images found increased human albumin in podocyte vesicles and on the apical membrane in nephrotic compared with control rats. Apocynin, an inhibitor of NADPH oxidase, decreased superoxide production in podocytes, and inhibited endocytosis and urinary albumin excretion. Real-time confocal microscopy found an initial delay in the appearance of Evans Blue-labeled human albumin in the tubular lumen, reflecting the time needed for transcellular transport. Immunoprecipitation analysis indicated that FcRn, a receptor for albumin transport, mediated podocyte albumin transport, and treatment with anti-FcRn antibody reduced proteinuria in these nephrotic rats. Thus, podocyte albumin transport was enhanced in PAN nephrotic rats by means of FcRn, which may explain the mechanism of selective proteinuria. This was blocked by apocynin, suggesting a new therapeutic approach.