The following parameters were measured in 63 renal biopsy specimens, most of which exhibited mesangioproliferative glomerulonephritis: the relative area of the interstitium and the area of the capillaries in the cortex and outer stripe of the outer medulla, and the area of the epithelium of the proximal tubules and of the ascending limbs of Henle's loop. The percentage of hyalinized glomeruli was also determined. Investigation of correlations between these values and parameters of renal function revealed the following: 1) The serum creatinine concentration increases and the endogenous creatinine clearance decreases significantly as the interstitium of both the cortex and the outer stripe of the outer medulla increases in width. 2) The urine osmolality decreases significantly as the epithelial areas of the proximal tubules and ascending limbs of Henle's loop decrease, and as the serum creatinine concentration rises and the areas of the interstitium increase. 3) No significant correlation exists between the percentage of hyalinized glomeruli and the urine osmolality. 4) The total area of the intertubular capillaries in the cortex decreases significantly as the interstitium in this area increases in width and as the serum creatinine concentration increases. 5) Compensatory hypertrophy of individual nephrons, as proposed by Bricker's hypothesis, was found only where more than 90% of the glomeruli were hyalinized.(ABSTRACT TRUNCATED AT 250 WORDS)