The effects of frusemide on glomerular permeability were investigated in eight proteinuric patients by measuring the fractional protein and dextran clearances and correlating these observations with changes in renal haemodynamics. 2. Frusemide increased significantly the fractional albumin and IgG clearances. The fractional dextran clearances of molecules with a radius greater than or equal to 5.4 nm also increased significantly after frusemide injection. Pretreatment with indomethacin partly inhibited the increments in clearances of macromolecules. 3. The changes in the fractional protein clearances correlated significantly with those of inulin clearance There was also a high degree of correlation between the changes in fractional protein clearances and prostanoid excretion. 4. The data obtained suggest that frusemide increases glomerular permeability by influencing the effective pores of the glomerular capillary wall. The increased permeability possibly is due to changes in prostanoid synthesis.
