1. The distribution and amount of ferritin in the glomeruli following intravenous injection of radiolabeled ferritin (125I-ferritin) was studied in 25 normal rats and in 25 rats with membranous nephropathy. The animals used were male Sprague-Dawley rats weighing 180-200 g at the beginning of the experiment. Membranous nephropathy was induced by repeated iv injections of 1.0 mg cationic bovine serum albumin during 28 days. 2. At the end of the experiment the animals received 125I-ferritin iv and were sacrificed 2, 6, 12, 24 and 36 h later, and the glomeruli were isolated. 3. Mean (+/- SEM) levels of 125I-ferritin in the glomeruli reported as cpm/mg protein in rats injected with cationic bovine serum albumin were: 731.8 +/- 155.6 after 2 h, 946.4 +/- 268.2 after 6 h, 565.4 +/- 143.5 after 12 h, 251.8 +/- 26.5 after 24 h, and 202 +/- 29.1 after 36 h. Mean (+/- SEM) 125I-ferritin in normal rats were: 2 h: 256.2 +/- 44.6; 6 h: 214.2 +/- 8.78; 12 h: 198.2 +/- 32.2; 24 h: 51.5 +/- 3.57; 36 h: 40.6 +/- 5.48. 125I-ferritin levels in the glomeruli isolated from rats injected with cationic bovine serum albumin were significantly higher than in control rats at 2, 6, 24 and 36 h. 4. The distribution of ferritin in the glomeruli was studied by a direct immunofluorescence technique. Normal and nephrotic rats showed ferritin in the glomerular mesangium only, with similar pattern and intensity. 5. These data show that rats with membranous glomerulonephritis induced by cationic bovine serum albumin presented an increased macromolecule uptake by the glomerular mesangium. However, the mechanism underlying this mesangial overloading is still unknown.