In experimental models of diabetes, glucose levels in plasma and blood are commonly determined by colorimetric assay and by automated analyzers based on the glucose oxidase conversion of glucose and O2 to gluconate and H2O2. We have compared the glucose levels obtained by these two methods in control Wistar rats, streptozotocin diabetic Wistar rats, Zucker fa/fa fatty rats and Zucker Diabetic Fatty rats. We found that the manual glucose assay and the glucose analyzer produced comparable values up to concentrations of about 25 mM. Above this level, samples should be diluted.