The antihyperglycemic effects of chemically synthesized d-chiro-inositol (d-CI), a component of an insulin mediator, have been demonstrated in rats. Buckwheat contains relatively high levels of d-CI: thus, it has been proposed as a source of d-CI for reducing serum glucose concentrations in diabetics. The present study evaluates the effects of a buckwheat concentrate, containing d-CI, on hyperglycemia and glucose tolerance in streptozotocin (STZ) rats. In fed STZ rats, both doses of the buckwheat concentrate (containing 10 and 20 mg of d-CI/kg of body weight) were effective for lowering serum glucose concentrations by 12-19% at 90 and 120 min after administration. Findings from this study demonstrate that a buckwheat concentrate is an effective source of d-CI for lowering serum glucose concentrations in rats and therefore may be useful in the treatment of diabetes.