Serum and urinary proteins from rats with nephrotic syndrome (NS) induced by puromycin aminonucleoside (PAN) were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Analysis was made on days 2, 4, 6, 8, 10, 12, 16, 20, and 30 after PAN injection. Data were compared with control rats (C). Rats developed proteinuria on days 4-30 and hypoproteinemia on days 4-16. Total protein concentration in serum and urine was similar on day 6. SDS-PAGE revealed that urinary albumin augmented on days 4-30 and serum albumin decreased markedly on days 4-20. Albumin concentration in serum and urine was similar on days 4-16. In addition, the study examined serum changes of 7 other proteins (designed as A, B, C, D, E, F, and G) which appeared or increased in urine, and whose molecular weights were higher (A, B, and C) or lower (D, E, F, and G) than that of albumin. In serum, protein A remained unchanged; protein B and G increased; proteins C, D, E, and F decreased. The qualitative pattern of urinary proteins remained essentially unchanged on days 4-30. During the intense proteinuria, the serum concentrations of protein B and albumin were similar and the urine concentrations of proteins C and D became comparable to that found in serum. These 7 serum proteins did not show the same behavior although all of them were excreted in urine. These data indicate that in PAN-nephrotic rats: (a) urinary proteins can be of low and high molecular weight, (b) serum proteins can be regulated independently of their urinary excretion and molecular weight, (c) the urine concentration of total protein and some specific proteins can reach values similar to that found in serum during the intense hypoproteinemia, and (d) the qualitative pattern of urinary proteins was unrelated to the magnitude of proteinuria.