Fourteen normotensive men (7 black, 7 white) were studied following equilibration during dietary sodium intake of 10, 300, 600, 800, 1200, and 1500 mEq sodium per day. Significant (p less than 0.05) increases in mean arterial blood pressure were seen after sodium intake of 800 mEq/d. Blood pressure increased at lower levels of sodium intake (800 mEq/d) and to a greater magnitude (21 mm Hg) in blacks than in whites (1200 mEq/d; 13 mm Hg). Sodium loading was associated with marked suppression of plasma renin activity, aldosterone and norepinephrine, and increases in cardiac index. At higher levels of sodium intake urinary potassium loss was seen. A subsequent experiment replacing urinary potassium losses as they occurred in six subjects demonstrated attenuation of the blood pressure increases seen in response to dietary sodium loading. These studies demonstrate a potential role for sodium and potassium in blood pressure regulation in normotensive man, and suggest that heterogeneity of response may be involved in the development of hypertension in individuals predisposed to avid sodium conservation.