The trace element vanadium exerts insulinlike effects in vitro and decreases hyperglycemia in insulindeficient animals. This study examined whether vanadate can improve glucose homeostasis in genetically obese hyperglycemic insulin-resistant ob/ob mice, which present metabolic abnormalities similar to those of human non-insulin-dependent diabetes. Sodium orthovanadate (0.3 mg/ml) was administered for 7 wk in H₂O. Vanadate treatment induced a fall in fed and fasted plasma glucose and insulin levels and improved tolerance to oral glucose; the stimulated glucose area was decreased by 65%, and an early peak of insulin secretion was restored. During an intravenous glucose tolerance test, the glucose disappearance rate was twofold higher in vanadate-treated mice, and the reappearance of a significant insulin response was also observed. Moreover, vanadate produced a twofold increase in hepatic glycogen content and prevented the exhaustion of pancreatic insulin stores. The hypoglycemic response to exogenous insulin was similar in control and treated mice. In vitro experiments showed that basal glucose oxidation by hemidiaphragms was 32% higher in vanadate-treated mice than in controls, although stimulation by insulin was similar in both groups. In conclusion, oral vanadate caused a marked and sustained improvement of glucose homeostasis in diabetic insulin-resistant mice by exerting an insulinlike effect on peripheral tissues and apparently preventing the exhaustion of pancreatic insulin stores.