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To investigate the regulation of arterial pressure and vasoactive hormones in phaeochromocytoma, we measured intra-arterial pressure and hourly venous hormones (renin, angiotensin II, aldosterone, catecholamines and cortisol) for 24 h in two patients before and three months after removal of a catecholamine-secreting tumour. Before surgery, plasma catecholamines, renin and angiotensin II levels showed a weak inverse correlation with arterial pressure, whereas after surgery norepinephrine-blood pressure correlations were close and positive. Heart rate and arterial pressure fluctuations were reciprocal during short-term recordings but were parallel after removal of the tumour.Renin, angiotensin II and aldosterone levels, initially elevated, fell into the normal range after surgery. Circulating norepinephrine correlated in a positive fashion with renin before and after operation. Slopes of log renin (or angiotensin ll)/aldosterone, and log cortisol/ aldosterone regression lines were steeper in the initial study compared to the second. We conclude that arterial pressure fluctuations measured hourly are not determined by concurrent levels of catecholamines or angiotensin II in phaeochromocytoma, although we surmise that minute-by-minute changes in blood pressure are catecholamine-related. Renin release is, at least in part, controlled by circulating catecholamines. Aldosterone responsiveness to angiotensin II and to ACTH is enhanced in phaeochromocytoma.