Matrix expansion in the glomerular mesangial area is observed in diabetic nephropathy. Intracellular breakdown of long-lived proteins was lower in mesangial cells in the high glucose medium than that in the control medium. Enzymatic activity of cathepsin L increased 1.4-fold after 6 h of treatment with the high glucose, and then declined gradually to 72% of control cells after treatment for 36 h. Change in the enzyme activity of cathepsin B showed a similar time course but less magnitude than that of cathepsin L. Immunoblot analysis with anti-cathepsin L antibody showed that change in the enzyme activity of cathepsin L was due to the change in the amount of cathepsin L, and that with anti-cathepsin B antibody showed no change in the amount of cathepsin B in the mesangial cells treated with high glucose. Intracellular cathepsin activities were controlled not only by the amounts but also by the inhibitor cystatin beta. Immunoblot analysis with anti-cystatin beta antibody showed that intracellular levels of cystatin beta increased slightly after 24 h of treatment with high glucose. These changes were derived from changes in mRNA level. These results, therefore, demonstrated that the decrease of intracellular protein breakdown in mesangial cells treated with high glucose medium was due to both suppression of cathepsins and increase of cystatin beta.