Many renal diseases are associated with fibrin deposition in the glomeruli, a situation that reflects an abnormality in the balance between the coagulation and fibrinolytic systems. We recently demonstrated that normal mouse kidney contains very low levels of type 1 plasminogen activator inhibitor (PAI-1), a potent anti-fibrinolytic protein, but that during endotoxemia, large amounts of PAI-1 protein and mRNA are expressed in glomerular and peritubular endothelial cells. These results raise the possibility that overexpression of PAI-1 in the glomerulus may contribute to the ongoing pathology seen in renal disease. To directly investigate this possibility, we studied PAI-1 expression in MRL/lpr mice, using in situ hybridization and immunohistochemistry. Female MRL/lpr mice develop early onset lupus glomerulonephritis (GN), a disease in which fibrin deposition is detected in the glomerulus and in which anti-coagulation therapy improves the prognosis. We detected very low levels of PAI-1 mRNA and antigen in the smooth muscle cells of renal vessels and in the renal papilla of 16 control mice. In contrast, PAI-1 was expressed in relatively high levels throughout the kidneys of 33 out of 34 diseased mice, both within the glomerulus and also in tubules and vessels. Moreover, the level of PAI-1 in the tissues seemed to correlate with the severity of the disease. PAI-1 expression was localized to endothelial cells, parietal epithelial cells, tubular epithelial cells and infiltrating mononuclear cells in the tubulointerstitium. None of these cells express detectable levels of PAI-1 in the normal kidney. The inappropriate expression of PAI-1 in the kidneys of mice with lupus GN suggests that this important inhibitor of fibrinolysis may play a role in the pathogenesis of this disease process.