1. In 91 essential hypertensive patients cumulative sodium balance was studied during the first week after they were taken into hospital. Sodium intake was fixed at 55 mmol/day.

2. After the first week, when sodium equilibrium was achieved, haemodynamic and endocrinological studies were carried out (cardiac output, renal plasma flow, plasma volume, measurements of plasma concentrations of total and active renin, aldosterone and noradrenaline).

3. On the basis of cumulative sodium balance a discrimination could be made between three groups. Group I (23 patients) accumulated sodium (52 mmol) before attaining equilibrium; group II (35 patients) were in balance from the beginning; group III (33 patients) exhibited a negative balance and lost on the average 125 mmol before equilibrium was attained.

4. On admission blood pressure and other characteristics were similar in the three groups. The decrease in blood pressure over the first 24 h period was equal. After that, the group characterized by a negative cumulative sodium balance (III) continued to exhibit a fall in blood pressure whereas groups I and II did not.

5. After sodium equilibrium had been attained, the haemodynamic profiles were somewhat different between groups. Group I (with a positive balance) showed greater systemic and renal vascular resistances than the other groups.

6. It is concluded that sodium loss contributes to the decrease in blood pressure on the second day of hospital admission and after. The blood pressure reduction during the first day is due to an independent mechanism, e.g. a decrease in sympathetic nervous tone.

7. It is also apparent that the attainment of sodium equilibrium during clinical investigations constitutes no guarantee that subjects are physiologically comparable; the preceding sodium balance has to be taken into account.

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