Body weight, urinary volume, sodium and potassium excretion, and blood pressure were evaluated for six days in twelve type II diabetic women, taking insulin doses over their need, after insulin daily dosage reduction. Six of them were suffering from essential hypertension. A significant decrease in body weight with an increase in urinary volume and in urinary sodium excretion was found in all patients. However, no significant variation in plasma renin activity and urinary aldosterone output was observed. Urinary potassium excretion remained unchanged. In normotensive subjects no variation of blood pressure levels occurred. On the contrary, hypertensive patients exhibited, after insulin reduction, a fall in systolic and diastolic blood pressure, which correlated directly with body weight decrease and inversely with urinary volume and urinary sodium excretion increases. Hypotheses relating insulin action to hypertension are discussed.