1. Anaesthetized cats were subjected to two 10 min trials of head-up tilting spaced 30 min. Arterial pressure, heart rate, blood flow to an innervated kidney and to the contralateral denervated one, and renin release from both kidneys were measured.
2. The same haemodynamic changes and comparable increases in renin release from innervated kidneys occurred during both episodes of tilting when cats were subjected to sham cervical vagotomy between the two tilting trials.
3. Bilateral cervical vagotomy, performed after the first episode of head-up tilting, did not affect the haemodynamic response to the change in posture, but significantly and markedly reduced the increase in renin release from the innervated kidney.
4. After sino-aortic denervation a marked and sustained arterial hypotension occurred during tilting, and the postural increase in renin release from the innervated kidney was even greater.
5. In no condition, even during the marked fall in blood pressure in sino-aortic denervated cats, did head-up tilting increase renin release from the denervated kidney.
6. It is concluded that maintenance of arterial pressure during tilting is mainly due to sino-aortic reflexes, whereas vagal reflexes are mostly responsible for the postural increase in renin release.