During nephropathy, the presence of proteins in urine of dogs is relatively frequent. The detection and characterization of proteinuria may be an important help to establish diagnosis and prognosis in renal diseases. In 50 dogs, 25 of which were healthy (healthy group) and 25 of which were with nepropathy (patient group), the urine proteins and proteinuria were evaluated by urine proteine to creatinine ratio of concentrations (UPr/Cr), sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. The UPr/Cr ratio concentrations was significantly increased in patient dogs in comparison to control group (0.558 +/- 0.425 and 0.041 +/- 0.026, p < 0.05 respectively) and 21 dogs of the patient group (84 %) had a UPr/Cr ratio above 0.2, indicating a pathological proteinuria. At least, 10 different bands were evidenced by SDS-PAGE and were divided into 3 groups: LMW (low molecular weight) proteins (below 66 kiloDaltons-kD), MMW (middle molecular weight) proteins (66-76 kD) and HMW (high molecular weight) proteins (above 76 kD). In patient group, 2 types of proteinuria were obtained : Glomerular tubular proteinuria characterized by presence of HMW or MMW and LMW in 67 % of dogs having a marked proteinuria and tubular proteinuria characterized by the presence of only LMW in 33 % of dogs. In healthy dogs, only scarce bands were visiualised by SDS-PAGE. By immunoblotting, transferrin, alpha(1)-microglobulin, beta(2)-microglobulin and retinol binding protein (RBP) were more frequently detected in patient dogs than in healthy dogs (p < 0.05). Moreover, RBP was detected in all the patient dogs with proteinuria, whereas it was detected only in the 2 of the healthy dogs, and this protein could be relatively specific for tubular damage.