The role of potassium in the control of blood pressure was first suggested from cross-cultural studies in which populations consuming high levels of potassium displayed lower rates of hypertension and a decrease in blood pressure with age. Cohort studies within this country have also shown a relationship between potassium intake or the ratio of potassium and sodium, as measured by urinary excretion and blood pressure. Most constantly it found that blood pressure differences between blacks and whites have been related to differences in potassium excretion.