The contribution of gluconeogenesis to hyperglycemia in non-obese diabetic (NOD) mice has been investigated using oral vanadate administration, Vanadate compounds have been shown to mimic many actions of insulin; however, the exact mechanism is poorly understood. The aims of the present study were (1) to elucidate vanadate's action in vivo, and to assess the possibility that its glucose-reducing effect is dependent on the presence of a minimal concentration of insulin; and (2) to evaluate the effects of vanadate administration on the key hepatic gluconeogenesis enzymes, glucose-6- phosphatase (G-6-Pase) and phosphoenolpyruvate carboxykinase (PEPCK), as well as glucose-6-phosphate dehydrogenase (G-6-PDH). Vanadate caused a significant reduction in blood glucose but failed to normalize it, despite effective serum vanadate concentrations (26.2 ± 1.6 μmol/L). Two weeks after initiation of treatment, blood glucose levels were 26.0 ± 1.8, 21.7 ± 3.0, 16.0 ± 1.6, and 14.3 ± 2.3 mmol/L in the control (C), insulin (I), vanadate (V), and combined Vanadate and insulin (V + I) groups, respectively (P < .001). G-6-Pase activity was significantly reduced by vanadate (622 ± 134 v 365 ± 83 nmol/min/mg protein in C v V, P < .05). PEPCK activity was also significantly reduced (844 ± 370, 623 ± 36, 337 ± 43, and 317 ± 75 nmol/min/mg in the C, I, V, and V + I groups, respectively, P < .001). No significant differences in the hepatic glycogen stores and G-6-PDH activity were noted between treatment groups. Our study suggests that the inhibition of hepatic G-6-Pase and PEPCK activity by vanadate plays an important role in reducing blood glucose levels in NOD mice. (C) 2000 by W.B. Saunders Company.