Abnormalities in albumin metabolism in nephrotic syndrome are still controversial. In an attempt to clarify the pathogenesis of the alteration in albumin metabolism in nephrotic syndrome. 131I-human serum albumin (131I-HSA) of 10 microCi loading was intravenously injected to 10 nephrotic (7 minimal change, 3 membranous nephropathy) and 3 non-nephrotic patients. Kinetic study was made by means of two compartment analysis following the 131I-HSA administration. The results were as follows: 1. Albumin synthesis decreased to some extent in nephrotic syndrome. 2. Absolute albumin catabolic rate decreased in nephrotic syndrome. 3. Fractional disappearance rate of albumin (FDR) and albumin turnover rate increased in association with progression of nephrotic syndrome. Both albumin escape into the extravascular pool and urinary excretion rate of albumin also increased. 4. In comparison of two types of nephrotic syndrome, minimal change and membranous nephropathy, it was found that, although renal and peripheral vascular permeabilities increased in both types, minimal change tended to show higher permeability than membranous nephropathy.