Total exchangeable sodium and blood volume were measured in 126 diabetic patients without renal or heart failure, in 52 patients with essential hypertension and in 99 normal subjects. Both body constituents were judged using the body surface area as the frame of reference. Exchangeable sodium (ExNa) was significantly increased in diabetics as compared to normal subjects (110 vs. 100%), regardless of age, sex or body habitus. ExNa averaged 108% in the 63 diabetics with normal blood pressure, 111% in the 63 hypertensive diabetics and 99% in essential hypertension. Blood volume (BV) was normal or sometimes slightly low in the diabetics with normal or high blood pressure and in essential hypertension. In normal subjects, ExNa or BV were unrelated to age or systolic or diastolic blood pressure. In the diabetic population, ExNa correlated with the systolic blood pressure (r = 0.25; p less than 0.005) but not with the diastolic blood pressure or age; BV was unrelated to blood pressure. The correlation between ExNa and systolic blood pressure was closer in the diabetics with high blood pressure (r = 0.47; p less than 0.001); this relationship could not be explained by a concomitant influence of age or plasma creatinine (rp = 0.34; p less than 0.05). These findings indicate that an increased ExNa associated with a normal circulatory volume is a common abnormality in patients with nonazotemic diabetes mellitus. Excess body sodium may play an important pathogenetic role in the maintenance of diabetes-associated hypertension.