A material with a molecular weight comprised between 10,000 and 50,000 has been extracted from the urine of normal and glomerulonephritic (GN) rats submitted to high or normal salt diet. When injected into the renal arteries of normal rats, the material isolated from the urine of normal salt-loaded rats induced a significant natriuresis; on the contrary, the material isolated from GN rats or from normal non-salt-loaded rats induced only a slight natriuresis which did not exceed control values. Injection of natriuretic material in GN rats did not induce any significant natriuresis. The results demonstrate the presence of a natriuretic material in the urine of salt-loaded normal rats; this material is absent in the urine of GN animals; moreover, they demonstrate that the sodium retention by the GN kidney is due to the inability of this organ to respond to the determinants of saline natriuresis such as natriuretic factor(s) as well as non specific changes in blood composition, i.e. blood dilution. Several possibilities could be deduced from these results: a natriuretic material of extrarenal origin could be inactive on the GN kidney which could destroy it; natriuretic material of renal origin could disappear during experimental glomerulonephritis and be absent in the urine of these animals.