Of all nutrients that may modify arterial pressure, hypertension is most convincingly related to dietary NaCl intake. In some experimental animals and in a significant percentage of hypertensive humans, the development of hypertension is dependent on a high dietary intake of NaCl (Horan et al. 1985). Although it is generally assumed that the effect of NaCl on blood pressure is specifically related to sodium, in 1904 Ambard and Beaujard reported that both urinary chloride excretion and blood pressure were reduced in hypertensive patients consuming a salt restricted diet. Their focus on chloride rather than sodium was related to the ease with which it could be measured. With the advent of techniques for measuring sodium, interest in NaCl dependent hypertension subsequently became focused on sodium. Several years ago, we initiated a series of studies to determine if the anion provided with sodium in the diet is important in the development of salt sensitive hypertension.