In 30 patients with a variety of glomerular renal disorders, intravenously administered furosemide was associated with a significant increase of the urinary excretion of the protein fraction with a molecular weight higher than 50,000. The excretion of protein with a molecular weight less than 50,000 was not altered. In relation to endogenous clearance of creatinine the clearance of transferrin, IgG and α<sub>2</sub>-macroglobulin increased significantly. In relation to transferrin clearance, the clearance values of albumin and IgG remained unchanged, whereas that of α<sub>2</sub>-macroglobulin increased significantly. These results support the view that furosemide increases the permeability of the glomerular capillary for serum proteins.