The mutarotase activity in the kidney, liver, serum, and urine of rats with experimental nephrotic syndrome induced by the intravenous injection of rabbit anti-rat kidney serum, was measured by our sensitive assay. It became clear that the mutarotase activity in the liver was almost constant, that in the kidney markedly decreased, and a considerable amount of mutarotase appeared in the serum and urine. It was therefore concluded that some amount of mutarotase in the kidney of rat was surely released into the serum and urine when the animal was suffered from nephrosis.