The vanadyl (+IV) form of vanadium has been demonstrated to have insulin-mimetic activity in vivo. In an effort to improve the poor gastrointestinal absorption of the ion, an organic complex of vanadyl (naglivan) was synthesized. We tested the antidiabetic effects of naglivan in rats made diabetic with streptozotocin (55 mg/kg, i.v.). Four days after the streptozotocin injection, one diabetic group (DVI) and a control group (CV) were treated with naglivan (50 mg/kg/day, equivalent to 0.06 mmol vanadium/kg/day) by oral gavage. Treatment in the DVI group was supplemented with daily insulin while a second diabetic group (DI) was administered daily titrated doses of insulin alone (Protamine Zinc, s.c.) to achieve stable euglycemia. The dose of exogenous insulin required to maintain normal glucose was significantly lower in the DVI group compared to the DI throughout the treatment period. At the end of week 3, exogenous insulin was withdrawn from both the DVI and DI groups, while naglivan treatment was continued in the CV and DVI groups for an additional 5 weeks. At termination, hearts were isolated and cardiac function (+dP/dt, -dP/dt and left ventricular developed pressure) was assessed in all the animals. After insulin was withdrawn, 4/8 DVI animals which continued to receive naglivan had consistent normoglycemia (as determined by % glycosylated hemoglobin) and an improved cardiac function. All the DI animals and 4/8 DVI rats were hyperglycemic and had depressed heart function despite having similar plasma insulin levels to the euglycemic DVI animals. As with vanadyl sulfate, there were no signs of long-term toxicity with regards to renal or liver function after 8 weeks of treatment. Thus, naglivan is an orally effective form of vanadyl with an oral potency 7.6 times greater than that of vanadyl sulfate (minimum effective dose: 0.06 mmol vanadium.kg-1.day-1) as compared to vanadyl sulfate (0.46 mmol vanadium.kg-1.day-1). The lack of incidence of diarrhea in either control or diabetic animals demonstrates that naglivan could be a more therapeutically desirable form of vanadyl.