1. A study has been made of urinary peptide output in rats before and after production of a Fanconi syndrome induced by a single injection of sodium maleate. There was an unequivocal increase of urinary peptides on the first and second days after the injection, without any detectable change in the concentration of plasma peptides.
2. Similar results were obtained in osteolathyritic rats in which skeletal lesions had been produced by ingestion of β-aminopropionitrile.
3. The fractional amino acid content of urinary peptides after maleate and β-aminopropionitrile is shown to be significantly different from that in control animals.
4. Evidence is presented that the increased output of peptides is mainly due to increased renal clearance similar to that previously described for amino acids, glucose and several electrolytes in this type of experimental Fanconi syndrome.