AbstractCareful ultrastructural studies of the rat model of nephrotic syndrome induced by puromycin aminonucleoside have demonstrated morphological features which are only seen in proteinuric glomeruli fixed without interruption of the blood pressure. These consist of balloon‐like swellings bounded by attenuated epithelial cell cytoplasm, with an area of bare basement membrane at the base. A theory of the mechanism of proteinuria was proposed on the basis of these findings. To test the proposed wide validity of that theory, we improved the method of surface fixation and performed similar studies in sequential manner, using chronic serum sickness glomerulonephritis in the rat as a model of proteinuria. Glomeruli were studied by light microscopy, transmission and scanning electron microscopy. The findings were correlated with the level of proteinuria in the 24 h preceding death and with the duration of serum sickness.Epithelial cell ‘balloons’ are also demonstrated in this model, correlating with the presence of proteinuria, but with slightly different configurations from those seen in puromycin nephrosis. Surface fixation revealed similar balloons in two other models of proteinuria: a graft versus host induced model of systemic lupus erythematosus in the mouse, and chronic streptozotocin‐induced diabetes in the rat.A lengthy search failed to find bare basement membranes in any of these models of proteinuria. We conclude, therefore, that the mechanism of proteinuria proposed in puromycin nephrosis does not apply in these models, and we suggest an alternative mechanism by which the ‘balloons’ may develop, as a further manifestation of the epithelial cell dysfunction which causes foot process