Abstract:Vanadate and vanadyl, two forms of vanadium, have been reported to exert insulin‐like effectsin vivoandin vitro.In the present study we compared the effectiveness of oral sodium metavanadate (NaVO3), sodium orthovanadate (Na3VO4) and vanadyl sulphate pentahydrate (VOSO4·5H2O) treatment in alleviating some signs of diabetes in streptozotocin‐induced diabetic rats. Vanadium compounds were administered in aqueous solutions of NaCl (80 mM) at concentrations of 0.20 mg/ml (NaVO3), 0.50 mg/ml (Na3VO4), and 1.1 mg/ml (VOSO4·5H2O) for two weeks. Control rats, either diabetic or non‐diabetic, drank solutions of NaCl (80 mM). Although some signs of diabetes (hyperglycaemia, hyperphagia, polydipsia) were significantly ameliorated by the vanadium treatment, negative side effects were also observed in all of the vanadium‐treated diabetic rats. Those effects included some deaths, decreased weight gain, and tissue vanadium accumulation, which are consistent with the reported toxicity of vanadium in non‐diabetic rats. Vanadyl sulphate was the most effective compound of those tested in normalizing blood glucose levels. However, the results here reported suggest that chronic administration of vanadyl or vanadate in the drinking water is not a viable alternative treatment to insulin in hum