1. Female Wistar rats were made heavily proteinuric by daily intraperitoneal injections of bovine serum albumin over 5 days.

2. The size and charge permeability characteristics of the glomerular filter in this condition were determined by studying the renal handling of poly-dispersed uncharged dextran over the range of molecular radii 2–6 nm (albumin 3.6 nm), and of endogenous rat serum albumin over the same clearance period.

3. In proteinuric rats the clearance of rat serum albumin was significantly increased with a resultant reduction in the circulating levels of rat serum albumin. The clearance of uncharged dextran was not significantly different in proteinuric rats compared with control animals.

4. There does not appear to be any size selective defect of the glomerular filtration barrier in this condition. This suggests that the observed increase in the clearance of the negatively charged endogenous albumin may be due to a reduction in the glomerular charge barrier.

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