1. The intrarenal infusion of concentrated urea after clamping of the renal artery produced immediate proteinuria in the dog. The predominant lesion on ultramicroscopy was destruction of the epithelial layer. Colloidal iron staining showed decreased fixed anionic charges in the capillary wall. 2. Sieving studies with neutral macromolecules such as 125I-labelled polyvinylpyrrolidone and [3H]dextran or an electronegatively charged polymer, [3H]dextran sulphate, showed a moderate increase in permeablility to the neutral tracers and a much more severe alteration of the lectrostatic barrier to the anionic polymer. The fractional clearance of dextran sulphate molecules increased to a greater extent than clearance of neutral dextrans of comparable size. 3. The shape of the curve relating the fractional clearance of dextran sulphate to molecular size is also modified in the contralateral kidney. This may be due to elevated plasma angiotensin II concentrations.