1. The level of proteinuria induced in female Wistar rats after bovine albumin injection intraperitoneally was highly dose dependent.
2. The proteinuria remained highly selective, with albumin constituting approximately 90% of the total protein excreted, even in the most severely affected rats.
3. Calculations relating the amount of bovine albumin available in the serum for filtration to the amount excreted in the urine indicated that complete saturation of the reticuloendothelial and tubular protein reabsorption systems-5-have occurred during the course of the 5 day injection period in rats given more than 3.5 mg of bovine albumin day−1 g−1 body wt.
4. When this situation was attained there appeared to be no further increase in glomerular permeability to either rat or bovine albumin and an equilibrium state seemed to exist where, when serum albumin levels were raised above the normal level, all the excess albumin passed across the glomerular filter to be excreted in the urine.
5. The passage of these large quantities of albumin across the glomerular filter-5-have resulted not from ultrastructural damage to the filter itself but rather from the generation of vastly increased concentration gradients across the glomerular basement membrane, which were sufficiently large to overcome the electrostatic repulsive forces which normally severely restrict albumin filtration.