1. A single intravenous administration of rabbit tonin antiserum into one-kidney one-clip hypertensive rats restored blood pressure to normal in seven out of ten animals. There was little change in blood pressure in two-kidney one-clip hypertensive, uninephrectomized or sham-operated rats.
2. Infusion of tonin in control rats did not modify arterial blood pressure. However, in indomethacin salt-treated rats a marked increase in arterial blood pressure was observed under tonin infusion.
3. Plasma tonin activity was significantly increased in human essential and renovascular hypertension.
4. These findings strongly suggest that tonin is important in the maintenance of high blood pressure. However, other factors (possibly prostaglandins and sodium) have to be modified in order to activate the tonin—angiotensin II system.