We have analyzed the protein content of proximal tubular fluid (PTF) by ultramicro disc electrophoresis and measured total protein excretion rates both in control conditions and during angiotensin infusion to the rat. Under control conditions PTF albumin concentration was 1.49 +/- 1.12 (SD) mg/100 ml and did not increase with distance from the glomerulus. Immediate postcapsular samples (Munich-Wistar strain) yielded nearly identical values so that both probably represent filtered albumin concentration. During infusion of angiotensin (0.15 mug/mix x 100 g of body wt), PTF albumin concentration increased on the average 26-fold in re-collections from control tubules. Total protein excretion increased from a control of 7.91 to 24.37 mg/24 hr x 100 g of body wt. Glomerular filtration rate (FGR), single nephron GFR (SNGFR), proximal transit time and tubular fluid to plasma (tf/p) inulin values did not change significantly. Net afferent filtration pressure decreased from 24.7 to 15.6 mm Hg and renal plasma flow fell from 2.16 to 1.31 mo/min x g of kidney wt. Data describe a protein reabsorptive system normally operating near capacity. Angiotensin-induced proteinuria derives from an increase in filtered protein (mostly albumin) resulting from permeability changes in the glomerular membrane.