1. Arginine vasopressin was infused into seven healthy young male volunteers at 12·5 and 25 units/min for 1 h at each dose. Plasma renin activity fell sharply and progressively in each subject. The mean fall was 47% and 66% of the initial value at the end of the lower and higher rates of arginine vasopressin infusion respectively; over 70% of the observed fall in plasma renin activity occurred at the end of the first infusion period.

2. The majority of the plasma arginine vasopressin concentrations achieved were within the physiological range observed after fluid deprivation and orthostatic stress in man, particularly at the lower rate of infusion.

3. There was no change of arterial pressure, despite a slight bradycardia at the lower rate of infusion; at the higher rate of infusion, there was only a very slight pressor response.

4. There was a concomitant and significant fall of plasma protein concentration and peripheral venous packed cell volume without any significant change of plasma sodium concentration or plasma osmolality, implying an expansion of plasma volume.

5. The results indicate that, in man, physiologically relevant amounts of arginine vasopressin suppress the rate of renin secretion indirectly by increasing the plasma volume at the expense of the extravascular fluid volume.

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