Albumin; andα1-,α2-,β-, andγ-globulins were estimated by cellulose acetate electrophoresis in the serum and urine from rats with nephrotic syndrome (NS), 2, 4, 6, 8, 10, 12, 16, 20, and 30 days after a single injection of puromycin amino-nucleoside (PAN). It was found that: (a) total serum protein level decreased on days 4–16, and total urine protein excretion rose on days 6–16; (b) serum albumin level fell on days 4–16, and urine albumin excretion increased on days 6–16; (c) serumα1-globulin level rose on days 8–30, and urineα1-globulin excretion increased on days 8–16; (d) serumα2-globulin level remained essentially unchanged, and urineα2-globulin excretion rose on days 4–10; (e) serumβ-globulin level decreased on days 4–20, and urineβ-globulin excretion increased on days 6–16, (f) serum y-globulin level diminished on days 6, 8, and 12, and urineγ-globulin excretion rose on days 6–10. All serum protein fractions were excreted in the urine of nephrotic rats; these findings suggest that proteinuria is nonselective. The differences observed in the serum protein profiles, even when all protein fractions were lost in the urine, suggest an independent regulation of each protein fraction in PAN-nephrotic rats. In addition, the electrophoretic profile of serum proteins in PAN-nephrotic rats is different from previously reported patterns in human nephrosis and in rats with an acute-phase response.