We wish to determine what cellular and functional alterations are associated with the development of glomeruloscierosis when rats with one kidney are fed an excess of salt or protein. Rats with one kidney are more likely to develop pronteinuria and glomerulosclerosis than control animals. Blood pressure recordings indicate that proteinuria and glomerulosclerosis occur before hypertension is evident. Fluorescent antibody studies disclose that albumin accumulates in the epithelial cells of glomeruli and tubules. Ultrastructural examination shows that vacuolozation of epithelial cells and basement membrane thickening precede the sclerotic collapse of capillary loops. Increased concentrations of sodium or urea that are found in urines of these rats favor the point of view that an elevation of solute load when combined with a reduction of renal mass will on some unknown manner accelerate the deterioration of glomeruli.