It has been shown by Newburgh and his collaborators1 that diets high in protein are injurious to the kidneys. In the attempt to explain the mechanism of this injury it was found that the intravenous injection of certain amino-acids into animals gave positive results,2 and that the injury produced was obtained by a single dose or, at most, by the administration of a few doses of the amino-acid in question. These acute lesions caused by large intravenous doses suggest the possibility that the introduction of the amino-acid into the body in small doses with the food over a long period of time might also produce injury. This investigation was devised to obtain an answer to this question. Cystine was selected as the amino-acid to be studied in this regard for several reasons: 1. When it is injected intravenously,2 a severe lesion is produced. 2. Cystine is an essential amino-acid, as