1. Nine normal subjects (eight male, one female) on a fixed daily intake of 150 mmol of sodium and 80 mnol of potassium, were randomized to receive either 3 days of 1.0 litre total water intake/24 h (food + fluid) or 4 days of 6.8 litres total water intake/24 h, and were then crossed over after a 3 day control period (2.7 litres water/24 h). 2. During water restriction, urine volume fell from 1.94 litres/24 h to less than 1 litre/24 h by the first day and was 0.77 litre/24 h on the final day. Plasma atrial natriuretic peptide levels were unchanged from baseline despite a large increase in plasma vasopressin and plasma and urine osmolality. Urinary sodium was unaltered throughout, while urinary potassium was increased on the final 2 days of water restriction. 3. During water loading, urine volume increased from 1.85 litres/24 h to 5.44 litres/24 h on the first day and remained at approximately 6 litres/24 h for the final 3 days. Plasma atrial natriuretic peptide showed no change. Plasma vasopressin and plasma and urine osmolality were reduced. Urinary sodium and potassium output were unchanged from baseline. 4. These results suggest that changes in plasma atrial natriuretic peptide are unlikely to be involved in the normal homoeostatic response to changes in water balance in man.

1. These studies were designed to investigate the effects of high dietary K + on electrolyte and water balance in young spontaneously hypertensive rats (SHR) and to relate these effects to changes in blood pressure. 2. The high K + diet reduced blood pressure by approximately 10 mmHg during the development of hypertension. Blood pressure, however, plateaued at the same maximum level as control by age 13 weeks. 3. Rats fed the high K + diet showed a significant increase in water intake and urine volume throughout the treatment period but no change in plasma volume or extracellular fluid volume occurred. 4. A slight natriuresis was also observed in rats on the high K + diet, but this was not of sufficient magnitude to decrease total body Na + . 5. These results confirm previous findings that K + causes a diuresis and a natriuresis, but demonstrate that the diuretic action of K + cannot explain its antihypertensive properties in young SHR.

1. Blood pressure was measured after treatment with a high K + , a low Na + and a combined high K + /low Na + diet in young spontaneously hypertensive rats (SHR). 2. A high K + diet reduced blood pressure by approximately 10 mmHg during the development of hypertension. This decrease was accompanied by a significant increase in water intake and urine volume and a significant decrease in plasma renin activity (PRA). 3. A low Na + diet also decreased blood pressure significantly, but, in contrast to the high K + diet, water intake and urine volume significantly decreased and PRA increased. 4. When both diets were given together, the antihypertensive effects of both were eliminated. Thus while an increase in dietary K + and a decrease in dietary Na + are both effective antihypertensive regimens in SHR, the mechanism of action of each appears to be different and may be antagonistic in these animals.