Mesangium enlargement is a constant feature of diabetic nephropathy and is likely to be important in the pathogenesis of this diabetic complication. Whether decreased degradation of mesangium plays any role in causing the enlargement is uncertain. We developed a system of preparing radioactively labeled mesangium matrix from mesangial cell cultures to be used as substrates for studies of mesangium degradation. Degradation is commenced by growing mesangial cells on the labeled matrix and monitored by the release of radioactivity into the culture medium. High glucose concentration (30 mM), whether present 1) when the matrix is being made or 2) when the degradation is taking place, reduces the rate of mesangium degradation. The second but not the first of these two phenomena was abolished by aminoguanidine. Phorbol 12-myristate 13-acetate, added in a manner to antagonize the action of protein kinase C, inhibited mesangium degradation and was not able to nullify the effect of high glucose. Thus it appears unlikely that a high glucose concentration inhibits mesangium degradation by increasing mesangial cell protein kinase C activity. We conclude that decreased degradation of mesangium as a result of hyperglycemia may play a role in causing the mesangium enlargement that occurs in diabetic nephropathy.