1. Twenty middle-aged men with untreated sustained essential hypertension for more than 5 years and 19 comparable normotensive controls were investigated. Both groups were derived from The Oslo Study, where they had served as control hypertensive and normotensive subjects. 2. Supine venous and arterial plasma catecholamines were increased in the hypertensive subjects compared with the normotensive subjects. The mean arterial--venous difference for adrenaline in the hypertensive (0.45 +/- SE 0.08 nmol/l) was increased compared with the normotensive group (0.27 +/- 0.03 nmol/l, P less than 0.05). Similarly, the venous--arterial difference for noradrenaline was increased in the hypertensive (0.29 +/- 0.13 nmol/l, P less than 0.05) compared with the normotensive group (-0.07 +/- 0.11 nmol/l). 3. The results are consistent with an increased release of adrenaline from the adrenal medulla and noradrenaline from the peripheral vascular beds (forearm) in essential hypertension. The increased arterial--venous difference for adrenaline in the hypertensive group also suggests an increased uptake of adrenaline in the peripheral vascular beds.