Sodium orthovanadate is one of vanadate salts. When administered orally to STZ-induced diabetic rats, sodium orthovanadate may stimulate glucose uptake and metabolism and made them normoglycemic. However, the blood glucose levels of normal rats treated with sodium orthovanadate did not change despite their plasma insulin levels decreased. The insulin-like effect of sodium orthovanadate on carbohydrate metabolism was not due to direct stimulation of insulin secretion, because the plasma insulin concentration of vanadate-treated STZ-rats remained lower and insulin secretion of isolated pancreas perfused with solution containing sodium orthovanadate was similar to that of controls.