1. Rats were made hypertensive by ligating the aorta between the origins of both renal arteries. Sham-operated animals served as controls. Urinary and renal kallikrein activities, as well as plasma and renal renin activities, were measured 8 and 90 days after surgery.
2. Blood pressure was 155 ± 6 mmHg on day 8 after aortic ligature and 142 ± 6 mmHg on day 90; in controls pressures were 107 ± 3 and 110 ± 5 mmHg respectively.
3. Eight days after aortic ligature, kallikrein activity in the ischaemic kidneys was about 6·5 times, and in the non-ischaemic kidneys almost 2 times, that in controls. After 90 days the kallikrein activity was reduced to one-half of that in the controls in the ischaemic kidneys and it was normal in the contralateral.
4. The urinary kallikrein excretion of hypertensive rats was about one-third of that of the controls at both 8 and 90 days after aortic ligature.
5. The plasma renin activity in hypertensive rats was approximately seven times that in control animals 8 days after aortic ligature and did not differ from the control value after 90 days. Renin activity in the kidneys showed the same pattern as in other models of renovascular hypertension: elevation in the ischaemic kidney and reduction in the non-ischaemic one.