The urinary excretion of the lysosomal hydrolases cathepsin B and beta-N-acetylglucosaminidase (beta-NAG) was compared with the tubular activities of these enzymes in remnant kidneys 16 weeks after subtotal nephrectomy (5/6 NX) or unilateral nephrectomy (UNX), as well as in kidneys from diabetic rats. In addition, the urinary excretion of the low-molecular weight protein cystatins, inhibitors of lysosomal cathepsins, was also followed in these animals. The urinary excretion of cathepsin B and beta-NAG was significantly enhanced in all three models of renal disease. The highest excretion rates for these enzymes were found in diabetic animals (cathepsin B: 4-fold; beta-NAG: more than a 10-fold increase over respective controls). In terms of tubular enzyme activities, tissue activities of both hydrolases were reduced in the remnant kidney after 5/6 NX, while in UNX and diabetes only cathepsin B activity was decreased. The urinary excretion of cystatins was enhanced in all three animal models, particularly in 5/6 nephrectomized rats, where a 40-fold increment over control animals was observed. Taken together, these findings indicate that there was severe tubular damage in the remnant kidney after 5/6 NX (reduced tubular enzyme activities, enzymuria and severely compromised tubular protein reabsorption). Furthermore, considerable enzymuria and disturbed protein reabsorption in early diabetes suggest tubular dysfunction before signs of glomerular damage become evident.