Female Wistar strain rats injected intraperitoneally with 1 g of BSA per day for 5 days, a total of 5 g of BSA, developed heavy proteinuria. Characteristic morphological abnormalities were seen by both TEM and SEM in the large majority of glomeruli from these animals and were similar overall, but not identical to, those seen in puromycin aminonucleoside nephrosis. The heterologous protein overload model seems to present considerable opportunity for study of the basic mechanisms of glomerular visceral epithelial cell handling of protein.