Although vanadium is found abundantly in animal and plant kingdoms its biological effects are not clear. Vanadate compounds have been shown to normalize blood glucose levels in streptozotocin treated rats, enhance glucose oxidation and improve the sensitivity to insulin by enhanced receptor binding in rat adipocytes. The aim of the present study was to investigate the effect of vanadate, at high (0-8 mmol l-1) and low (0-1.0 mmol l-1) physiological concentrations, on [125I]-insulin binding in the placenta of three groups of patients, namely from normal (N) controls, gestational diabetics (GDM) and women with risk factors in their medical history for developing diabetes mellitus (RF). Vanadate at low concentrations (0.2-0.6 mmol l-1) enhanced the maximal binding 2-fold in GDM placenta but only increased (up to 1.2-fold) the binding slightly at high concentrations (5 mmol l-1). However with placenta from normal or women at risk, vanadate increased the [125I]-insulin binding up to 1.2-fold both at low and high concentrations. Thus it appears that vanadate augments insulin binding in the placenta from women with gestational diabetes mellitus.