Renal protein mass increases in diabetic renal hypertrophy. Accretion of protein may be the result of increased protein synthesis and/or decreased protein degradation. The lysosomal proteases, cathepsins B and L, are key enzymes in cellular protein catabolism. To evaluate the role of protein degradation in diabetic renal hypertrophy, the activities of cathepsins B and L were measured in microdissected proximal tubule segments and in kidney cortex homogenates. In rats four and ten days following induction of diabetes by streptozotocin, the kidney weight was increased and the cathepsin activities were reduced in proximal tubule segments. Treatment with insulin prevented both changes. The liver weight in diabetic rats was decreased and the activity of cathepsins B and L was increased, while the activity in kidney cortex was reduced. This excluded that diabetes per se may be accompanied by decreased cathepsin activities independent of organ hypertrophy. Renal hypertrophy as a cause rather than as the consequence of reduced cathepsin activities was excluded by the finding of unchanged cathepsin activities in proximal tubule segments from rats with compensatory renal hypertrophy four days and ten days following unilateral nephrectomy. Decreased activities of cathepsins B and L may reflect decreased intracellular protein degradation. Decreased protein breakdown in proximal tubules may contribute to diabetic renal hypertrophy. In agreement with this interpretation are the results from rats six months following induction of diabetes. Renal hypertrophy is complete at that time. No further accretion of protein occurs and the cathepsin activities in the proximal tubule were not different from controls.