1. In eight normal subjects, ten patients with labile hypertension, six with advanced essential hypertension and six with renovascular hypertension, plasma renin concentration, cardiac output, mean arterial pressure, clearances of creatinine and p-aminohippurate (PAH), and sodium excretion were measured before and after 30 min of 45° upright tilting. Changes in plasma adrenaline and noradrenaline concentration were measured in addition in the normal subjects, and in plasma volume in normal subjects and patients with labile essential hypertension. 2. In patients with advanced essential hypertension, heart rate and calculated peripheral resistance increased significantly less than in normal subjects, and plasma renin increased by 15% in this group, in comparison to 37% in normal subjects, 48% in labile essential hypertension, and 57% in renovascular hypertension. There was a positive relationship between changes in renin and noradrenaline concentrations in normal subjects. 3. Apart from a negative correlation between the increases in plasma renin concentration and mean arterial pressure in patients with renovascular hypertension, there were no significant relationships between changes in plasma renin and haemodynamics. 4. A negative correlation between changes in plasma renin and filtration fraction and a positive relationship between changes in renin and sodium excretion were found in normal subjects and patients with labile hypertension. Plasma renin increase was directly related to changes in the tubular rejection fraction of sodium in patients with labile hypertension. In the same group there was a negative correlation between changes of sodium rejection fraction and filtration fraction. 5. The results suggest a role of the adrenergic system in orthostatic renin release, but the functional connection between renal haemodynamics, tubular sodium handling and renin release across orthostasis cannot fully be explained on the basis of our present knowledge of renin releasing mechanisms.