Background: It has been well known that GTP-binding proteins (G-proteins) are involved in insulin actions. So, in order to evaluate whether the G-protein is involved in the pathogenesis of diabetes mellitus, we evaluated the subcellular distribution and functional changes of G-proteins in adipocytes from streptozotocin-induced diabetic rats and changes in amount and function of G-protein in adipocytes from patients with diabetes mellitus. Method: To induce the diabetes 30 male rats were injected with streptozotocin (55 mg/kg I.v,). After 7 days, isolated adipocytes from control and diabetic rats were fractionated into homogenate (HOM), plasma membrane (PM), high density microsome(HDM), low density microsome (LDM), mitochondria (MIT), crude nuclear fraction (CNF), and cytosol (CYT). The isolated adipocytes obtained from lower abdominal subcutaneous adipose tissue of control and diabetic patients who underwant elective abdominal surgery were fractionated into homogenate and postnuclear fraction. The Gi1a & Gi2a, Gi3a, Gsa, and Gb were quantitated on Western blots in frations of rats. The Gi1a & Gi2a was quantified on Western blots in fractions of control and diabetic patients. In order to evaluate the functional changes of G-proteins pertussis toxin-catalyzed ADP-ribosylation and GTPrS binding activity of G-proteins were performed. Result: In adipocytes from streptozotocin-induced diabetic rats, the Gi1a&Gi2a, Gi3a, Gsa, and Gb proteins were mainly located in plasma membrane. Specifically, Gi1a & Gi2a, and Gi3a were enriched in plasma membrane of adipocytes from streptozotocin-induced diabetic rats. In contrast, pertussis toxin-catalyzed ADP-ribosylation of Gi proteins in plasma membrane of adipocytes from streptozotocin- induced diabetic rat significantly decreased. The GTPrS binding activity of G-proteins in plasma membrane of diabetic rat significantly decreased. To some extent, it was restored by insulin treatment. Although, there was no consistent change in the amount of Gi1a & Gi2a, pertussis toxin-catalyzed ADP-ribosylation of Gi-protein were consistently decreased in adipocytes from patient with diabetes mellitus. Conclusion: These results suggest that insulin-deficient diabetic state induces the quantitative and functional changes in Gi-proteins of adipocytes plasma membrane. Therefore, these changes in Gi- proteins may be an important compensatory reaction for the insulin resistance occurring in insulin deficient state.