OBJECTIVE This study evaluates the effects of dietary salt restriction and loading on blood pressure in normotensive and hypertensive patients with non-insulin-dependent diabetes mellitus (NIDDM). Salt sensitivity of blood pressure responses is compared to vascular reactivity to infused angiotensin II on the two sodium diets. METHODS We studied 19 patients with NIDDM (seven normotensive, 12 hypertensive) and seven nondiabetic control subjects under metabolic balance after 6 days on a constant low- (20 mEq) sodium diet and again after 6 days on a high- (250 mEq) sodium diet. Salt sensitivity of blood pressure was determined by the increment in integrated 24-hour blood pressure values on changing from the low- to the high-sodium diet. Blood pressure and plasma aldosterone responses to a graded-dose infusion of angiotensin II were also examined on both sodium diets. RESULTS Eight of 12 hypertensive patients with NIDDM displayed salt-sensitive blood pressure responses, whereas none of the normotensive patients with NIDDM or control subjects were salt-sensitive. Patients with NIDDM also had augmented blood pressure responses to infused angiotensin II on both sodium diets when compared to control subjects. Whereas controls had reduced vascular responses to angiotensin II on the low-sodium diet, these responses were not decreased in patients with NIDDM. Patients with NIDDM also retained more sodium on the high-sodium diet than did the control subjects. CONCLUSIONS Hypertension in patients with NIDDM is frequently salt-sensitive, which may be due to sodium retention and enhanced vascular reactivity to angiotensin II. Since sodium restriction does not normally reduce vascular reactivity to angiotensin II in NIDDM, salt-restricted diets may be less effective in blood pressure control in hypertensive NIDDM.