Different doses of anionic and cationic albumin (CA) were administered intravenously to 29 normal unanesthetized female rats. Administration of 20 and 30 mg of CA produced an increase in urinary excretion of endogenous albumin. Urinary CA levels in the urine samples collected during the first 60 min after injection of 30 mg of CA were higher than anionic albumin levels. A marked increase in the number of endocytic vacuoles, large numbers of images suggesting fusion of vacuoles with lysosomes, extrusion of cell elements into the tubular lumen and tubule rupture were observed in the animals injected with CA. These results show that CA may produce changes in proteinuria and in renal structure.