Many studies have shown that diabetes mellitus is associated with increased whole and blood viscosity and decreased erythrocyte deformability. It has been suggested that these abnormalities in blood rheology may play a causative role in the pathogenesis of diabetic vascular complications. However, less is known about the content and quality of membrane proteins which may contribute to abnormalities in membrane dynamic and decreased erythrocyte deformability. In the present study we analysed various rheological parameters (blood and plasma viscosity, erythrocyte deformability, haemotological parameters), in cats with non-insulin dependent diabetes mellitus (NIDDM). We also investigated alterations in erythrocyte membrane protein content by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). We found that erythrocyte rigidity and plasma and whole blood viscosities were significantly higher in cats with NIDDM compared to controls. SDS-PAGE revealed that the band 5 corresponding to actin was weaker while band 4.5 corresponding to integral membrane proteins (glycophorin A, B and C) had disappeared. Also, band 4.9, which is composed of dematin (a protein with actin-bundling capacity) was lost. We suggest that the observed abnormalities in membrane proteins may play a role in reduced erythrocyte deformability associated with diabetes mellitus. © 2005 - IOS Press and the authors. All rights reserved.