Serum and urinary concentrations of NCI, the non collagenous globular domain of collagen IV, were used as markers for turnover of basement membranes. NCI levels were studied in membranous glomerulonephritis and diabetic nephropathy. Thirteen patients with membranous glomerulonephritis and 8 insulin-dependent diabetic patients with diabetic nephropathy were compared to 16 apparently healthy control subjects. The patients with membranous glomerulonephritis had lower levels of NCI in serum and urine compared to the control subjects. In comparison, the patients with diabetic nephropathy had similar levels of NCI in serum and urine as the control subjects. Furthermore, among patients with membranous glomerulonephritis, those with hypertension had higher serum levels of NCI than those without, which may indicate that hemodynamic factors influence the basement membrane collagen metabolism. It is suggested that there are differences in basement membrane turnover in membranous glomerulonephritis and diabetic nephropathy although there are similarities in glomerular histopathological features. Other possible mechanism are discussed. Further studies are needed to confirm the suggested mechanism.