The possible relationship between mesangial dysfunction and development of glomerular sclerosis was studied in the puromycin aminonucleoside (PAN) model. Five male Wistar rats received repeated subcutaneous PAN injections; five controls received saline only. After 4 weeks the PAN rats were severely proteinuric (190 +/- 80 mg/24 hr), and all rats were given colloidal carbon (CC) intravenously. At 5 months glomerular sclerosis was found in 7.6 +/- 3.4% of the glomeruli of PAN rats; glomeruli of the controls were normal. Glomeruli of PAN rats contained significantly more CC than glomeruli of controls. Glomeruli with sclerosis contained significantly more CC than non-sclerotic glomeruli in the same kidneys. CC was preferentially localized within the sclerotic areas of the affected glomeruli. Since mesangial CC clearance from the mesangium did not change during chronic PAN treatment, we conclude that this preferential CC localization within the lesions is caused by an increased CC uptake shortly after injection in apparent vulnerable areas where sclerosis will develop subsequently. Cluster analysis showed a random distribution of lesions in the PAN glomeruli in concordance with the random localization of mesangial areas with dysfunction in this model. Similar to the remnant kidney model in PAN nephrosis the development of glomerular sclerosis may be related to "mesangial overloading."