In order to determine arterial pressure and vasoactive hormone relationships in normotensive man, we measured intra-arterial pressure continuously along with hourly venous hormone levels (renin, angiotensin II, aldosterone and catecholamines) for 24 hours in 5 healthy volunteers under standardized conditions. Mean 24-hour levels of intra-arterial pressure 106/63 +/- 5.4/4.9 mmHg were much lower than in patients with mild essential hypertension studied earlier. A common diurnal pattern was seen for plasma renin, angiotensin II, and catecholamines, with higher levels in the day time and lower levels at night. Aldosterone levels however, paralleled those of cortisol at night. Plasma norepinephrine levels showed close, positive correlations with arterial pressure in all volunteers. We conclude that the level of blood pressure as measured continuously over 24 hours is lower than might be expected from regular clinic recordings; that aldosterone regulation is contributed to by ACTH in the nocturnal hours; and that fluctuations in arterial pressure and sympathetic activity over 24 hours are closely coupled.