Six patients with essential hypertension underwent a randomized cross over design study to investigate the effect of supplementing a 10 mmol/day sodium diet for a period of 5 days with either 120 mmol sodium chloride (Slow Sodium, Ciba, Horsham, UK) or 122 mmol sodium in the presence of other anions, mainly phosphate (Phosphate, Sandoz, Feltham, UK). With both sodium salts, urinary sodium excretion was increased. The calculated amount of sodium retained was similar for both the sodium chloride and sodium phosphate periods. However, with the addition of sodium chloride to the low-salt diet, there were increases in supine mean blood pressure whereas with the addition of sodium phosphate no change in mean blood pressure occurred. The supine mean blood pressure after supplementation with sodium chloride (119.8 +/- 4.3 mmHg) was significantly greater than that after sodium phosphate (113.3 +/- 4.5 mmHg), similarly, the standing mean blood pressure was greater after addition of sodium chloride than of sodium phosphate (122.3 +/- 4.20 versus 115.4 +/- 3.0 mmHg). With both salts there were similar but non-significant increases in weight and reductions in plasma renin activity (PRA) and plasma aldosterone (PA).