The purpose of this study was to investigate whether intensive insulin treatment of dogs suffering from type 1 diabetes mellitus, resulting in tight glycemic control, could be reflected by changes in peripheral leukocyte metabolism. Specifically, plasma metabolites and enzyme activities were assessed. In addition, quantitative RT-PCR was used to determine changes in insulin signaling gene (insulin receptor substrate (IRS)-1, IRS-2 and phosphatidylinositol 3-kinase (PI3-K) P85alpha) mRNA levels in peripheral leukocytes. Lastly, leukocyte enzymes involved in cellular energy metabolism were examined for changes in glucose utilization. Our results indicated that intensive insulin treatment was successful in type 1 DM dogs, leading to tight glycemic control. The mean glucose concentration and glycated albumin percentage significantly decreased to 156 mg/dl and 15.6%, respectively, following treatment. In peripheral leukocytes, the IRS-2 and PI3-K p85alpha mRNA levels significantly increased, and a significant increase in pyruvate kinase and pyruvate carboxylase activity, two enzymes involved in cellular energy metabolism, was also observed post treatment. Therefore, the observed changes in insulin signaling pathway activity and cellular energy metabolism enzyme activity in peripheral leukocytes are considered to be characteristics of amelioration of glucose metabolism by insulin action. As such, peripheral leukocytes are sufficiently sensitive to monitor for improving glycemic control during intensive insulin treatment of type 1 DM dogs. Blood cells such as leukocytes are much more readily available than muscle or adipose tissue for studies in dogs.