We measured plasma fibronectin levels by a rocket immunoelectrophoresis in rats with chronic serum sickness induced by repeated injections of ovalbumin and in rats with epithelial nephropathy induced by a single injection of adriamycin. In the early phases of the immune model, rats presented granular deposits of lgG in the mesangial area with no or descreteproteinuria (<40 mg/24 h). Fibronectin levels in that group were significantly higher (450±90μg/mL) than in normal rats of the same age (350±46; p<0.01). When animals presented IgG deposits in the capillary wall, an important nephrotic syndrome developed in most of them. Fibronectin levels then increased very significantly (863±153μg/mL; p<0.0005). In the model of adriamycin nephropathy, fibronectin significantly increased (580±110μg/mL; p<0.0005) from the first week, when proteinuria was in a range 40–60 mg/24 h. However, the levels were higher (860±175μg/mL; p<0.0005) when a complete nephrotic syndrome developed. At this time, plasma fibronectin levels correlated directly in both models with the degree of proteinuria and inversely with the total serum protein concentration. Our results show that plasma fibronectin levels increased very early in animals with immune and toxic damage of the kidney. The highest elevated values found thereafter, when a full nephrotic syndrome was present, suggest an increased synthetic rate of that glycoprotein linked to that situation.