In the renal glomerulus, two extracellular matrices have been identified, the glomerular basement membrane and the mesangial matrix. Accumulation of glomerular extracellular matrix is a conspicuous feature of most forms of progressive glomerular disease, including diabetic nephropathy. Since proteoglycans are prominent components of the extracellular matrix, we examined the glycosaminoglycans and proteoglycans synthesized in vitro by mesangial cells from normal and diabetic rats. A mixture of dermatan sulfate and heparan sulfate was recovered. Dermatan sulfate was the predominant glycosaminoglycan synthesized and most of it was released to the culture medium, in contrast to heparan sulfate which was found to be cell associated to a higher degree. The dermatan sulfate chains are composed by D-glucuronic and L-iduronic acid-containing disaccharides and are highly sulfated. Mesangial cells from diabetic rats produce much more glycosaminoglycans than mesangial cells from normal rats, especially dermatan sulfate and this increase was proportional to the duration of diabetes. In contrast, exposure of mesangial cell from normal rats to elevated glucose did not lead to any changes in glycosaminoglycan synthesis, indicating that this short-term culture conditions may not adequately simulate diabetes mellitus. Other factors related to diabetes environment may be responsible for the observed alterations. The dermatan sulfate was secreted to the medium as proteoglycan. Two dermatan sulfate proteoglycans were identified, with molecular weights of 120 and 85 kDa respectively. The proteoglycan core protein M(r) was 45 kDa and the dermatan sulfate chains were 35 kDa. It is possible that the two proteoglycans represent two populations, one with two dermatan sulfate side chains (120 kDa) and the other with only one side chain (85 kDa), presumably fitting in the decorin/biglycan family of small proteoglycans.