1. Ten patients on maintenance diuretic treatment received an intravenous infusion of antidiuretic hormone at a low rate for 1 h.
2. A gradual reduction in mean plasma renin activity was observed and this was significant at 60 min.
3. There was a significant correlation between the initial value and the extent of the fall in plasma renin activity. There was no consistent change in blood pressure, heart rate and blood volume.
4. The results point to an intrarenal site of action of antidiuretic hormone.