To investigate the role of potassium on blood pressure we measured serum potassium, urinary excretion of potassium and sodium, fractional excretion of potassium, urinary sodium:potassium ratio, plasma renin activity, aldosterone, and norepinephrine during dynamic maneuvers in normotensive and hypertensive subjects. After baseline measurements, we expanded intravascular volume with infusion of intravenous saline and then induced sodium and volume depletion by diuretic administration during a low sodium salt diet. These studies were performed in 431 normotensive and 478 hypertensive subjects enabling evaluation of the effects of age, race, and sex, as well as blood pressure, on the results. Among normotensives, we found that white subjects had significantly P less than 0.05) higher levels of serum and urine potassium, fractional potassium excretion and lower urinary sodium:potassium ratios than black subjects and that males had the same patterns of differences compared to females. Similar, but less consistent racial differences were seen among the hypertensive subjects. We also observed significant (P less than .05) correlations between urinary potassium excretion and body weight in both normal and hypertensive groups. In normal subjects, a significant correlation was observed between the urinary sodium:potassium ratio and blood pressure that was not seen in the hypertensives. The latter, however, displayed a significant (P less than .05) inverse relationship between serum potassium and blood pressure. Multiple regression analysis revealed that urinary potassium excretion was influenced by age, race, sex, body weight, blood pressure, creatinine clearance, renin, and aldosterone. These observations reveal important relationships between potassium homeostasis and blood pressure control that deserve further study.