Oxidative stress parameters; thiobarbituric acid reaction substances (RBC-TBARS), catalase (RBC-CAT) and reduced glutathione (RBC-GSH) and the intraerythrocytic concentrations of electrolytes; sodium and potassium (RBC-Na and RBC-K) were determined in 18 well- controlled (WC) and 22 poorly-controlled diabetic mellitus (DM). Dogs with DM had significant higher blood glucose concentration (P < 0.001), haemoglobin A1c (P < 0.01) and fructosamine (P < 0.001) compared to normal healthy dogs (n = 19). Diabetic dogs in both groups had higher RBC-CAT (P < 0.05) while RBC-TBARS were higher significantly only in poorly-controlled DM group (P < 0.05). The RBC-K was significantly higher in both DM groups (P < 0.001). No changes in RBC-GSH and RBC-Na were found between DM and control healthy dogs. By linear regression analysis, the relationship were found between degree of diabetic mellitus and RBC-CAT, RBC-TBARS, RBC-Na and RBC-K. The relationship was also found between oxidative stress parameters and intraerythrocytic K+. The results suggest that in diabetic dogs, oxidative stress occurs which related to the severity of disease and may affect potassium homeostasis.