We studied the following sources of nitrogen in uremic patients: proteins, amino acids, urea, and molecular nitrogen. Four uremic subjects and one control were given diets containing normal or low amounts of proteins or minimal quantities of essential amino acids as nitrogen sources. Their diets were supplemented with urea 15N and sucrose 14C. Of the 18 amino acids studied, glutamic acid alanine, aspartic acid, serine, and glycine were labeled significantly more than the others. (1) By feeding uremic patients an essential amino acid diet, a positive nitrogen balance was acheived together with a gradual decrease of urea levels in the blood. (2) Labeled molecular nitrogen administered subcutaneously to a patient with pyelonephritis was later incorporated into protein to a significant degree.