1. Changes in plasma renin activity, plasma noradrenaline, pulse rate and blood pressure after tilting were measured in normal subjects and in patients with renal transplants.
2. There was a marked difference between the renin responses in the two groups, the increases in plasma renin activity in the control subjects being much greater than those in the transplanted patients.
3. Activation of the sympathetic nervous system after tilting, as indicated by changes in pulse rate, blood pressure and plasma noradrenaline, was similar in the two groups.
4. We conclude that the ability of the transplanted kidney to increase plasma renin activity after tilting is impaired, probably as a result of sympathetic denervation of the kidney during transplantation. The results suggest a dominant role of the sympathetic nervous system in the mediation of renin release after tilting.